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.

- 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outdated definitions.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

The former outline was always displayed square on the iPad

Ignore grid sizes when dynamic mode is active.
This has the added benefit of adding some actual dynamicality to the 
grid in the editor.

This is done according to the Angular best practices and minimizes
namespace collisions when using other packages which might use the same
selectors.

Technically the selectors are not even becessary since we use an Angular
component factory to create element components but it is still useful
for the DOM element naming, for debugging purposes for example.

The 2 different radio button variants (with and without images) are now 
hidden behind one radio button entry. Functionally everything else works 
the same.
Also move CSS to it's own file.

The package was not a proper ECMA Script package and therefore can not 
be tree shaken properly and would unnecessarily increase the package 
size.

- Simplified the code logic. Verification is done once in the beginning 
and not on every element.
- Now uses semver to verify and compare version strings

Now shows an error message and continues with an empty unit.

This may destroy alignment of inputs and produce unwanted scrollbars.