- 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outdated definitions.

Remove the wrapper divs in each element component and add the
'fixed-size-content-wrapper' class to the element overlays
of player and editor

The previous solution, which uses a top position of 50%, did not work on
iPad

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.

Elements are supposed to be centred when this setting is used. Since it 
is the same CSS for almost all element components it is put into global 
styles.

Missing components: video, spelling.

Section was picking up the size of children elements. With relative it 
works.

This allows elements in dynamic sections to have fixed dimensions.

Some modifications to property panel logic, to show dimension fields and
proper labels.

Only implmented in dropList and button elements.

- Replace onClick method with an event emitter inside the template

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.

The problem that was to be solved is that buttons within a form always 
cause form validation for all form elements. Which is not wanted here.
The better solution than preventing all event is to simply declare the 
button type. As button without any type are treated as submit buttons.

This allows to use them in template bindings. Used in compound elements.

- Remove interfaces
- Add classes for all elements as well as unit, page and section
- 2 intermediate interfaces are kept. They dont constituate a viable 
object but only additional properties, like background-color. Those can 
be added as interface to all elements. Initialisation has to be done by 
the element itself though and not by some constructor higher up.

This replaces the label text with an image, which can be uploaded.

* When elements receive the focusin event, their status is changed to
`TOUCHED`
* Replace `this.form.valueChanges` subscription with
`this.unitStateService.unitStateElementCodeChanged` subscription

This is in preparation for dynamic layouting.
The actual dimensions are set by the overlay component which differs but 
is existent for both editor and player.

Also refactor a lot of stuff belonging to this:
- You can edit the default values via properties panel
- All elements now again have a common parent directive. form elements
have a special parent for forms.

* Only elements which has to be registered by the player's form inherit
from `FormElementComponent`
* Element components use their specific `elementModel`
* Rename file for `FormElementComponent`

The UIComponents now only have the properties they can make use of. This 
means that the angular components use the CSS properties explicitly 
instead of having all kinds of useless CSS properties.
Also it is no longer needed to update the CSS styles via event because 
Angular's change detection takes care of that.

This makes the actual component elements agnostic to any drag and drop 
and selection logic, which is handled completely via the 
wrapper-overlay. Now the component elements can be easily used by the 
player.

They will be reused by the player.