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

- Some very basic tests of creating an element and changing some 
properties
- Reference check for button with page navigation

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

Shows which versions of Player and Editor belong together and which unit 
def version they interface with.

This whole class will be deleted soon.

The ClozeDocument needs properly instantiated UIElements for the custom 
elements.
Instead of creating a new ClozeDocument on the basis of what TipTap is 
producing and replacing the custom parts with instances, the 
ClozeDocument is kept in place but the custom parts are collected into a 
list and changed there.

#450