- Remove all Roboto files

Since there is just one font, this setting can be safely ignored. This 
is easier than sanitizing all elements.

#513

#499
- Problem was due to changes in the latest minor material update

#533
- Remove unnecessary and wrong setting of underline property

#531

#532
- Replace the global label identifier

#535

#454
- does not work in FF

#456

- Add Property "hasAutoHeight"
- Add Directive AutoHeight

#520

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

#524

- 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

- 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

#521

#521

#521

#521

- Replace or remove no longer used material customization

#521

- Remove no longer used material customization

#513

- Can also be used with other elements

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.

#512

- Remove dynamicPositioning from PositionProperties. This is not 
knowledge that the element needs to know. In the few places it was used, 
it was fixed or solved by asking the containing section about which 
positioning is active.

- Fix and improve cloze child sizing. All can now have dynamic width and 
height.

- Fix empty lines in ClozeDocument rendering

- Editor SelectionService now knows if a cloze child is selected. This 
way the PropertiesPanel can show valid dimension inputs regardless of 
the underlying section.

Also restructure files containing interfaces.

Octal characters are deprecated and don't work anymore. This solution 
uses an empty string over the whole line.

#502