#542

Also restructure files containing interfaces.

- Use scss extension fpr styles file

This is necessary to avoid playing parts of audios and videos twice
when a unit is reloaded, since audios and videos send their playback
time only every 5 seconds when they are played back.

- Remove prefix 'on'

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.

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 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outdated definitions.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.

- 'hideOtherPages' is not supported by verona api
- 'uninterruptible' is already fulfilled by disabling pause and
progress bar

... a played audio file

The interaction can now be turned off without hiding the button.

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.

Hint and autostart functionality of videos and audios is initialized
only after dependencies to other audios and videos have been resolved.
This prevents the hint of an audio and video from starting, although
the audio is not yet enabled.

Setting on type button avoids that click events are evaluated from
the surrounding form

In ControlBarComponent the property project replaces the property
isAspectPlayer. It ensures that delays are used only in the player
and hints are always visible in the editor.

All elements now use the elementValueChanged event. Also
formValueChanged is changed.

* Change the detection of the parent element to determine whether the
audio/video player is running in the aspect-editor or the aspect-player

* 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