ImportModuleVersion stores the unitDefinitionType of the loaded unit
definition. During the initialization of the unit, the individual
elements can react to the stored value.

and DropListSimpleElement

Use singleton instance of IdService in UIElement

This is not a complete backwards compatibility. Only the value "0" can
be safely classified as false. It is therefore set to "1".

- Refactor findNestedUIElement as static class method

Now correctly creates child elements for every marker it finds.

Updated the TipTap extensions to use model instead of just the ID.

Also improve interface naming.

Prevent sending a valueChange notification

Change creation of the list of possible drop list values.
The values can be found at different hierarchy levels in the unit
definition, as they can be sub elements of cloze elements