- remove ID handling from model classes. This is now done by the editor 
itself (mainly unit service).
- Rename and move IDManager to editor as IDService

Use map and flat instead of reduce to set dragNDropValueObjects

- Use Unit's getAllElements method of Unit instead of
UnitUtils.findUIElements to get all DragNDropValueObjects
in UnitComponent
-Remove unnecessary timeout in UnitComponent

- Sanitize without using unit definition version

- Add "isOlderThan" method to the "SanitizationService" to distinguish
old ToggleButtons with the property "option" from new ones
- Use SanitizationService.isUnitDefinitionOutdated in Player to set
the unitDefinitionVersion (should be modified!)

Fix import order

This button helps the user to detect if a page has additional,
hidden content

DragAndDropValueObjects are now a sub-interface of TextImageLabel, which 
requires a set imgSrc and imgPosition property. (The reason is that the 
editor needs a way of generating Labels therefore needs the imgSrc set 
to something else but undefined.)

- Refactor subscriptions for verona commands

- Use done event of keyboard animation to set the scroll position
of the element
- In small browser views elements are scrolled to the top of the view
instead of the middle of the view

- problem with nullish values