- 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

The logic determined the circumstances when min and max values were 
used.
Now they are passed as they are and the CSS ruleset does the rest.

In Player-context the constructors are to fail, when the given 
properties are not exactly as expected. Therefoore they are validated. 
When this validation fails, an environment variable is checked to 
determine if the element should be created anyway with defaults. This 
switch is off in player-context. But the Editor is not so strict and can 
create elements with incomplete parameters.

There is also a new VersionManager which checks given unit definitions 
for version-validity. Upon failing this check Player and Editor show 
error messages.
The Editor accepts a certain unit definition version, but shows a 
warning.

The Editor checks units for invalid references after loading. If any are 
found, they are removed and an info-panel is shown.