The updateListener does not detect all changes inside a GeoGebra element

#309

Also set geometry element to always use fixed size, which is all that 
works anyway.

Otherwise, no elements can be dropped at this place

To detect if a custom form element was touched it is necessary to set a
custom tab index

... to create new references for the connectedTo array.

Use the source list placeholderIndex instead of the index for the target 
list.

Chromium does not allow DOM manipulation on dragstart. Therefore add 
timeout.

Duplicate value objects without keeping the reference to the old object.

- Replace Material Droplist with native HTML events
- Remove simple-drop-list element; Cloze elements now use the normal 
DropList element
- Add example units

- TextAreaElement
- TextFieldElement
- TextFiledSimple
- SpellCorrectElement

- 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.