see commit #866a33c6

- Units and element are now built via the factory. This ensures only the
  wanted properties are present. All others are discarded.
- Since the sanatizer is supposed to check the unit defintion version
  it is used by player and editor and therefore put in common.
  This in turn makes it necessary to put the JSON resolver settings
  in the root-TSConfig file.
- Renamed a couple of properties to have clearer names.

Remove manipulations from elementModel
- Refactor drop list component
- Use of initial values for audio and video playback time

Defining the validators not in the elements anymore, but when creating
the forms

This helps to divide the code of the element container into smaller
meaningful units.
In addition, each element can be given its own form group.

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outdated definitions.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.

When a page has longer scrollable dynamic content that consists of many
individual elements (Likert), the content was sometimes not displayed
after interacting with those elements. This happened in the maximized
browser display, especially in Firefox.
Probably due to the deep dynamic nesting the height to be displayed
could not be calculated correctly

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.

Use the status of each element

* Add IntersectionService
* Add CompoundElementComponent as parent for LikertComponent
* Inject ElementRef to common ElementComponent
* Rename players' ElementComponent to ElementContainerComponent

* Remove FormService Injection
* Move FormService and form types to player
* Move interface ValueChangeElement to uI-element.ts
* Use interface InputElementValue instead of
string | number | boolean | null

* Take over tasks of `UnitStateComponent`
* Manage property `unitStateElementCode`
* Removing the passing of the property `unitStateElementCode` by
the components

* `UnitState.dataparts` has property `elementCodes` property, which
stores status, value and id for FormElement
* When building a unit, the stored value is assigned to the element
when it is initiated
* Removed `UnitState.dataparts.pages`
* Removed methods initPages and addStoredValues

* After deleting validation component, element component is no
longer needed and is deleted
* Element overlay takes over the methods of element component

* Fix scrollbar bug
* Remove using of timeout method
* Add Material fonts

* Remove unnecessary styles
* Refactor case distinctions
* Improve styles of some elements

Refactor styles for elements and element overlays to achieve stretching
of dynamic sections when text areas are resized

* Increase height for textarea and textfield after adding new material
styles
* Remove position for validations

* undo setting overflow hidden for sections
* set display of `ElementOverflow` to block

To center the content of the player, the dimensions of all pages are
calculated.

* Move two different intersection detections from
`scroll-index.directive` and `section.component` to
`intersection-detection.directive`

* Fix bug for small pages
* Add functionality for tabbed pages
* Use layoutContainer as container for intersection detection
* Rename properties and methods