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.

This prevents the browser from trying to translate the page on
German systems

Move required styles and classes to the overlays of player and editor

- Use fixedSize always together with dynamicPositioning!
- Rename css class 'center-content' to 'fixed-size-element'
'fixed-size-element' ensures that the content does not overflow.

This removes all the classes and goes back to simple interfaces for
unit, pages etc and all elements. This makes creating them easier and
cleaner.
Things like specific needs when setting element properties and most
importantly compatibility handling is done by a special service module,
that knows all the elements and how to repair old their potentially
outdated definitions.

Remove the wrapper divs in each element component and add the
'fixed-size-content-wrapper' class to the element overlays
of player and editor

Note that autostart of audios and videos does not work for tablets

The previous solution, which uses a top position of 50%, did not work on
iPad