- Use own StateVariableStateService which uses the same new base class
as UnitStateService

#525

#525

#506

#523
- Improve sanitizing of position margins

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

#524

#522

#452

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

Not just when an older version is detected.

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