Options still had a margin-top, which is wrong when there is no label 
above.

Fix typo

The theme file is now project specific because Angular demands assets
to be part of the project.
It also differs in content: The editor theme is only applied to canvas
elements. For the player every checkbox and radio button is affected.

The components now again use the default color of 'accent'. The custom
theme now has the green as accent color and no longer primary.

Used in checkboxes and radio buttons, which are now green.
The secondary is unused but for safety set to the same as the default
theme should be.

This new setting makes child items space around in a row with line 
break.

Images in vertical alignment now always reserve the whole row and don't 
appear next to each other.

- Rename NumbersKeyboardComponent to MathKeyboardComponent
- Decrease the number of input properties
- Split source code into private methods

Use unit field of UnitService directly to detect changes.

- Ignore blank pages when calculating the response progress

- Replace onClick method with an event emitter inside the template

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.