- Refactor findNestedUIElement as static class method

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