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.

This is a native input element which is used in cloze elements. Here 
(sub)elements need to be positioned inline, which is hard to do with 
material components.

Also add a method to all form elements to set their form value, which is 
now only used by the simple text field but may be useful for other 
elements, which don't integrate in material form fields.

Highlighted (background colored) text is done with mark elements which
don't have a color and make the contained text lose the styling of the
parent span, which has the text-color property.
To solve it we simply inherit the color of the parent span.

When using borders deselecting would remove the borders of the 
(text)element as well.

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.

The control bar is shown only when at least one control is selected
in the editor.

The tooltip declarations does not need escaped text. Was shawing one to 
many backslashes.

The issue here is the alignment if dropLists and text fields. They are 
supposed to be exactly in line with the text which is very complicated 
with Material components, which are heavily nested and contain several 
sub-elements which need to be changed - removing margins, paddings and 
other position related styles.

Min-height should be optional for dynamically positioned elements, as 
you usually don't need it and it causes more problems than it solves. 
Elements with min-height have cause unexpected margins between elements.

This avoids duplicate scrollbars in TestStudio

- Don't use height 100%. The image is not centered but positioned
in the upper left corner

As requested by product owner.

Now scales, taking up all available space.

No unused space around images and image an text below each each other.

This reverts commit b5d3b810.

The old behaviour with non-scaling images is wanted here. Will improve in subsequent commit.

Extend the parsing to inludes h-Elements.

The explicit check for undefined ensures that the value 0 of a
number property is taken from serialized elements.

Start could be a restart, so lists containing references to elements
are cleared.

If no audios and videos are available, the query status is 'complete'

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

This allows to set the background color without limiting the content

The presentation will not be completed until all audios with the
minRun property have been played.

Before the parent elements (span, div...) were catching the drop event 
which is not intended.

The player is supposed to take all the elements already defined and 
generated in the editor. Therefore the call to generate all elements 
from the text is not a general thing but specialized for the editor.

Right now the editor does no longer generate the cloze parts on load but 
only when the text prop has been touched.

Also all elements now take the parentForm as input. Allowing it to use 
them with template binding and not only setting the value directly (in 
the controller).