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

- Using scrollIntoView with target 'center' was not always reliable
  especially in FF

- The workarounds for ff with setTimeout are no longer necessary in the
  new version and have been removed
- Replace ChangeDetectorRef.detectChanges with setTimeout because the
  scroll button was not always reliably displayed at startup

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

- add hotspot image element
- raise version to 3.8.0

Rework LogLevels and adjust log statements

Level 2 is now the level with mild logging. Level 3 does not log 
anything apart from warnings and errors.

- 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

Options are now possibly rich text. This needs to be taken care of.
Also use extra span tag to preserve all whitesoaces in the index-suffix.

#279

Top and bottom were mixed up.

#291

- 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.