- Using the Geogebra method 'getBase64' is very resource intensive.

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.

- Make sure that the textarea has been displayed once (AfterViewInit)
  before its dimensions change again

- The width of the border is no longer needed because
"box-sizing: border" is used
- Scaling of the inner triangle areas is replaced by the setting of 500%
 width and height to be able to fill the whole area of the obtuse
 triangles and at the same time not change the width of their borders

- Also simplify the button constructor

- 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 deps only used by geometry to the geometry class and out of the 
parent.

DomSanitizer was a relict and no longer in use.

- Fix element creation to have new value objects instead of referencing 
the old one
- Add new IDs to DropList value objects

- overflow of lists is now hidden to prevent dropable elements from
being displayed outside the container
- placeholders are hidden with flex orientation

The unit tests errored out because of circular dependencies via the 
Elementfactory class. Therefore:
- No longer creates styling/position/player-Properties. This is done in 
the Element parent class.
- Move element creation from Section to the ElementFactory class 
instead.

Create new pipe for checking if element is included in list.

- Make position props on buttons optional
- Move UIElement creation back to Factory instead of within Section

Use a horizontal aligned placeholder for dynamic aligned drop lists
when there is more than one element in the list

- Refactor variable of the placeholder dimensions

- Important for vertically positioned drop lists
- Analogous to the calculation of the height

- Replace pixel calculation with percentage values
- Use of only two modes: full fill or line

#291

#343

Red background looks really bad, so we just adjust the border color.

- remove ID handling from model classes. This is now done by the editor 
itself (mainly unit service).
- Rename and move IDManager to editor as IDService