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

Change creation of the list of possible drop list values.
The values can be found at different hierarchy levels in the unit
definition, as they can be sub elements of cloze elements

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.

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.

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

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.

- This behaves analogously to the floating marker bar

- Remove element as parameter

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.

- Implement marking buttons as toggle buttons

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