- Basic implementation

Also restructure files containing interfaces.

- Part of the properties panel for every element (like delete and 
duplicate)
- This allows interacting with elements, generating events and inpect 
their usage. Usually this is unwanted since it might interfere with 
dragging the overlay, but sometimes you want to get a feel for the 
element or check it's contents (GeoGebra for example)

- A HotspotImage has an image and divs placed on it, which are used
as hotspots
- Inherits from FormElementComponent, which is why the hotspots are kept
  as a list in the value property.
- Can be validated as required and uses its own custom validator

- Move props to separate files
- Fix typing

Remove 'any's be adding proper types and using 'never'.

This way elements can handle their logic themselves without having to
rely on outside utility classes.

Also restructure files in common in a more logical way.

Components with bigger scope are put to the component root, canvas, 
props panle and new-element-panel.

Change import paths of CommonModule files in editor and player

The LikertColumn is actually just an image-text-combo. Has a generalized 
name now.
The other interface was not needed anymore and has been deleted.

This helps identifying the selected element.

- Use sub components to reduce the size of the templates
- Introduce field sets for input groups like dimensions and position
- Improve logic to determine when to show some fields

- Unify font and surface props
- Use intersection type for additional properties
- Also rework editor props panel to properly create the combined 
properties recursively, passing the results to its child panel (tabs).
New algorithm works without lodash, therefore the package is removed.

This removes all the classes and goes back to simple interfaces for
unit, pages etc and all elements. This makes creating them easier and
cleaner.
Things like specific needs when setting element properties and most
importantly compatibility handling is done by a special service module,
that knows all the elements and how to repair old their potentially
outdated definitions.

This is done according to the Angular best practices and minimizes
namespace collisions when using other packages which might use the same
selectors.

Technically the selectors are not even becessary since we use an Angular
component factory to create element components but it is still useful
for the DOM element naming, for debugging purposes for example.

The copy is needed to not have cross references between elements.

Was not correctly creating new instances of non-primitive values. Now
uses "JSON.parse(JSON.stringify(element))" for deep copying the element
first.

Also refactor by moving the method to the UnitService, where all ID
handling should take place.

Delete and duplicate buttons are no longer part of the tab view but are 
always visible below it.

Use a copy of the original elements to avoid accidentally changing them.

The change of moving some properties to sub objects broke the creation 
of the combinedProperties object, which can't deal with nested objects. 
To circumvent that all elements are now flattened before being combined.
The connected property panels now again look for the properties on the 
element itself and not in sub-objects.

- Dnd values now have a special type that has an ID, a string and
  another string for an image path.
- A new dialog exists for editing thoe options. Similarly to the likert
  header.

The problem being solved is that compound-sub-elements must not have
all the properties other elements have. For exampe positioning info.
Sub-Elements are positioned inline. To avoid having similar class
inheritance structures for positioned and inline elements, we use
interface. Positioning is done via interface and is therefore optional.

Those optional properties are kept in an object in a variable. This
allows to hide specifics from element classes and removes the need to
initialize all the fields in every class using the interface.
The elements mostly care about their own specific properties anyway.

The same technique is used for font and surface elements. Here we also
don't want to initialize all the styles in every class.

Changing values of element properties works like before. Just set the
property as if it would be a direkt prop of the element. The element
itself puts the value in the currect sub-object (positioningProps,
fontProps etc).

For reading the values there is a similar method, but it is not used
anywhere by now. Since the properties panel operates on the raw values
anyway (refer CombinedProperties) and for element components this would
mean calling a function in the template, which caused change detection
to run the function very often and is therefore bad practice.

Additional notes and refactorings:
- Restructure all files in common. UIElements in the same folder etc.
- Move all interfaces to the UIElement file. This avoids circular
  imports, which would be needed to allow PositionedElement to extend
  UIElement.
- Interface initializers also handle reading values in the old form.
- Add PositionedElement for use in canvas overlays. This is a UIElement
  with guaranteed postionProps.
- Don't export all Material packages from common, only the ones
  used in player and editor. The rest is only used in common and
  does not need to be exported.

No longer needs the appropriate section, but instead loops through all 
page sections. It was somewhat complicated to know the corresponding 
section and this solution solves that. Though at the price of some 
unnecessary element lookup.

Another upside is that elements from different sections can be deleted 
simultaneously.

Move the first tab to it's own component. File still is way to long and
unwieldy.

This structure moves closer to the structure template the team decided
on. Components and services are kept in dedicated folders instead of
following the logical code structure. This is still not the case here as
the unit-view component is still nested, but it's closer.

This change mainly helps the editor.
The idea is to make the element components have the correct value of
their model and don't rely on the formcontrol value. The formcontrol
can be hard to manipulate, especially in compound elements. Setting
the model value is way easier.

The formcontrols are kept and should not interfere.

This element has a list of string as editable options. Those strings 
form draggable elements which may be reordered or moved between 
different DropList elements.

Also refactor option change logic to allow any property not just the 
text property.

This is necessary to reset elements like radio button group. We use null 
for that instead of undefined.

The check was in place before to make sure input fields only accept 
valid values (no text in a number field for example). Angular thinks a 
null value is valid so we have to do this extra check. This is now done 
directly in the template before calling the updateModel method.