Remove useless plugin and add rule overrides for e2e test dir.

Variable naming, reorder imports etc.

Introduce interfaces and clean the structure.

This makes for cleaner imports.

The dropListList generation is now done when things are changed via the 
section menu and no longer on element update via unit service.
Also remove some interfaces to avoid circular imports. The interfaces 
are so small and insignificant that their form might as well be 
specified at usage point.

No longer have unit as subject. Components can use the unit variable as 
is and still get updated.
This removes some listener stuff in conected components.

Was not correctly creating new instances of non-primitive values. Now
uses "JSON.parse(JSON.stringify(element))" for deep copying the element
first.

Also refactor by moving the method to the UnitService, where all ID
handling should take place.

No longer is a singleton but a normal servicce now again. The singleton 
logic was needed in the past but not anymore.

Generally only the editor needs to generate new IDs. Before there was
need to have it in common because some elements (like compound elements)
needed to generte new Elements including IDs.
Now all ID stuff is handled by the UnitService and the elements classes
are handed the fully built elements. Unfortunately this means that some
logic has to move back from classes into the UnitService. To un-bloat
the UnitService a (and in the future some more) helper class is created
for this logic.

The most important thing is, that the UIElement no longer generates
an ID when created. The ID has to be created before and passed to the
constructor.

Add the missing property playerProps

The status of a page is defined by the state of its elements.
Therefore, after restoring the elements, a check of the status of a
page must be triggered.

Now correctly sets the chosen background color and additionally sets 
another (configurable) color to the selected value.

The other  variants are basically the same as filled. The only
difference is the background color, which we are manilupating anyway.
So there is no reason to keep them.

Also for the background color directive different target elements are
needed for the different appearance variants. This is now possible.
As stylings of the old appearance are kept when changing the appearance
the styles need to be removed first.
In the moment the appearance value is changed the component is not
actually changed yet and there we need to wait for another Angular
change detection cycle, via timeout.

- Use the same font properties for input element and button element
- Change cross out behaviour. The button now behaves as
toggle button and sets the focus to the input element only when it is
crossed through.

This is meant solely as a cloze child as a representation of a radio 
button functionality.

This allows to select elements without clicking them on the canvas, as 
elements may be obstructed or not visible for some reason.

The selection logic is a bit complicated because usually only the 
overlay components are usd for selection which are also the place to 
apply any highlighting on. Here we have to find the component element 
reference from the section. Therefore the double looping 
'getElementComponent' method.

Also when an elements is selected it briefly gets z-index 100 so it is 
moved to the front and visible and manipulatable.

For all elements using radio buttons.