This was necessary after the surrounding element container with its
styles and classes was deleted

Remove manipulations from elementModel
- Refactor drop list component
- Use of initial values for audio and video playback time

Defining the validators not in the elements anymore, but when creating
the forms

This helps to divide the code of the element container into smaller
meaningful units.
In addition, each element can be given its own form group.

- Now does unit (page) manipulation itself instead of delegating 
everything to the UnitService. Still need to call the UnitService to 
make it notify the API about the change to the unit.
- Also improve the hack for refreshing the tab view.

This makes for cleaner imports.

It did not do anyhting anyway. Is now integrated in unit view.

Since the array of 'ClozeElements' can be empty, it must be checked

Add margin between field sets.

Set back to the default setting to prevent CSS settings to spill over 
into other parts of the application.

The factory was ignoring the src parameter, which is set before by the 
UnitService.

Instead, use :host in conjunction with ::deep

- Use sub components to reduce the size of the templates
- Introduce field sets for input groups like dimensions and position
- Improve logic to determine when to show some fields

Instead, use :host in conjunction with ::deep

- Unify font and surface props
- Use intersection type for additional properties
- Also rework editor props panel to properly create the combined 
properties recursively, passing the results to its child panel (tabs).
New algorithm works without lodash, therefore the package is removed.