#533
- Remove unnecessary and wrong setting of underline property

#531

#532
- Replace the global label identifier

#535

#506

The individual rows are considered as a whole and are therefore
adjustable as a whole via the parent element

#454
- does not work in FF

#461
- This property will be used for all input assistance presets
  in the future

#456

- Add Property "hasAutoHeight"
- Add Directive AutoHeight

#520

#506

Make player control bar inherit pointer-event styling. This way the 
Editor can set it to none, while acting normal in the Player.

#524

#452

- Don't report empty reference string as invalid
- Add VideoElements to check

Absolute position didn't work if the display scaling factor of the
operating system or browser is not 1.

#511

GGB elements without toolbar aren't loaded, if this parameter is true

#511

- crossOutChecked adds line-through styling if checked. This is used in 
Cloze texts to cross out words.
- This means Checkbox is no longer a PositionedElement and does not 
validate PositionProperties

This way the unit definition version is set to current if an old version 
is detected (by the Editor).

#521

#521

#521

#521

- Replace or remove no longer used material customization

#521

- Remove no longer used material customization

Instantiate the child elements coming from the editor (i.e. when the 
document changes). Otherwise the cloze children are all on the same 
object reference.

#513

- Can also be used with other elements

This whole class will be deleted soon.