The directive for dynamic backgroudn color was outdated. Directive is 
removed and problem is solved via CSS custom variable.

#561

#449

- position
- label
- icon

Was completely broken: not showing at all in Editor, not clickable in 
Player.

Removed pipe 'aspectInputBackgroundColor'. Replaced by more elegant 
solution via inherited background-color.

THis component was still somewhat broken after the Material update.

#541

#545

The inclusion of svg icons via the src parameter of an image tag
is prohibited in the test centre for security reasons

- 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