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outdated definitions.

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.

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

This may destroy alignment of inputs and produce unwanted scrollbars.

It is better to hide that abstraction in the unit service.

and DropListSimpleElement

Use singleton instance of IdService in UIElement

Regenerate IDs for child elements. For this we use the child element 
list we have to produce for the player anyway. To be able to use it, it 
has to move to the element though.

Only the overlay can be clicked. This way the statre of the elements can 
not be changed which might be confused for actual value changes. Those 
can still only be done via properties panel.

Unfortunately we need another variable so the cloze elements knows when 
it runs in the editor or not.

Do not change the underlying temp object, but instead use an new object 
reference.
This avoids changing references to objects of other elements.

All elements which report the selected index are supposed to send an 
index starting with 1 instead of 0. This a special requirement for the 
analysis of answers.

Since we can't safely assume any other font than the one we ship is 
available, it is better to not allow changing this.

I have not removed the variable completely as in the futire more than 
one font might be wanted. So for now you just can't choose another.

Also focus the height input field upon opening the menu.
For the layout menu this does not work as checkboxes don't play nicely 
with menus.

In fact having anything but button in the menu is against the Material 
spec. But i still think this is the best way to achieve the usability we 
want.

For dynamic section we keep the solid line as the dashed one is harder 
to see. Needs to be discussed maybe.

Input elements are now grouped in fieldsets.
They also a cool gradient background color.

Compound elements are now supposed to use overlays for their child
elements. This overlay makes selecting child components (by clicking on
them) and marking them as selected (done by the selection service)
possible.
The SelectionService no longer needs special logic to handle compound
children selection, as they now also have an overlay with the same
interface as normal (canvas) elements.

A few modifications in connected directives are necessary. The now
handle children components in a proper array instead of QueryList.

Likert elements do not have clickable children yet and work a little
differently. This should probably be unitized in the future.

Lower and the input element panel does not show properly.

This way the dialog does not grow too wide, when lots of text is 
entered. There could be a lot more logic here to calculate exactly how 
big the window should be, but that seems like overengineering right now.

- 'hideOtherPages' is not supported by verona api
- 'uninterruptible' is already fulfilled by disabling pause and
progress bar

Refactor MathKeyboardComponent to reduce code size

Was not working at all before. Now the logic is much simpler with the 
use of tiptap attributes.

Was broken after previous refactoring.
The renderHTML method only works with a proper return. Don't know why. 
Therefore disregard the linter warning for now.