- Areas are marked with the surrounding 'aspect-anchor' tag

- If this property is set, the input of more characters than specified
in maxLength is prevented
- It also prevents the insertion of text.

- Replace Material Droplist with native HTML events
- Remove simple-drop-list element; Cloze elements now use the normal 
DropList element
- Add example units

- Rewrite interface 'VopMetaData'
- Add parameter 'metadata' to VopReadyNotification

- Use the VeroSubscriptionService as a pure communication service
- Do not contaminate the VeroSubscriptionService with holding data

- Using scrollIntoView with target 'center' was not always reliable
  especially in FF

- The workarounds for ff with setTimeout are no longer necessary in the
  new version and have been removed
- Replace ChangeDetectorRef.detectChanges with setTimeout because the
  scroll button was not always reliably displayed at startup

- A HotspotImage has an image and divs placed on it, which are used
as hotspots
- Inherits from FormElementComponent, which is why the hotspots are kept
  as a list in the value property.
- Can be validated as required and uses its own custom validator

- 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.)