Use singleton instance of IdService in UIElement

This is not a complete backwards compatibility. Only the value "0" can
be safely classified as false. It is therefore set to "1".

Now correctly creates child elements for every marker it finds.

Updated the TipTap extensions to use model instead of just the ID.

Also improve interface naming.

Prevent sending a valueChange notification

Use a border with border-radius set to make it look nicer.

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.

This way the placeholder is exactly as big as the font size. There is no 
space for a padding here, as the normal dropList has.

Frames are supposed to be used to group other elements and therefore 
usually needs to be behind them.

- Remove element as parameter

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.

- Implement marking buttons as toggle buttons

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

Name changed during development

Do not consider all form elements, but use a list containing only form
elements with validators

Refactor MathKeyboardComponent to reduce code size

Is now able to interpret more stuff from the editor. The only thing 
missing are the list styles.