Setting on type button avoids that click events are evaluated from
the surrounding form

This allows to set the background color without limiting the content

The presentation will not be completed until all audios with the
minRun property have been played.

Before the parent elements (span, div...) were catching the drop event 
which is not intended.

The player is supposed to take all the elements already defined and 
generated in the editor. Therefore the call to generate all elements 
from the text is not a general thing but specialized for the editor.

Right now the editor does no longer generate the cloze parts on load but 
only when the text prop has been touched.

Also all elements now take the parentForm as input. Allowing it to use 
them with template binding and not only setting the value directly (in 
the controller).

The font size is applied by setting the list style. This is a 
workaround, since the list extension does not support setting font size 
directly. It takes the selected font size (which is now saved ina 
variable) and applies it. This is indepent of the font size of the text 
in the list. The item font size is done individually with a span anyway.

Also font size and list style droplist now (re-)apply the value on click 
of an item. This allows updating font sizes and list styles without 
having to select another option first.

- Add a new variable to save the selected font size and reuse it for 
list styles.

(when not set)

No longer needs the appropriate section, but instead loops through all 
page sections. It was somewhat complicated to know the corresponding 
section and this solution solves that. Though at the price of some 
unnecessary element lookup.

Another upside is that elements from different sections can be deleted 
simultaneously.

This reverts commit c71cf1a2.

Child elements need some special CSS, for example to prevent taking up 
more space than needed.
This is only for text field and drop list, which are the relevant sub 
elements for now.

Button is no InputElement which causes trouble in some places. Since it 
does not make much sense in the Cloze context it is just removed.

The problem that was to be solved is that buttons within a form always 
cause form validation for all form elements. Which is not wanted here.
The better solution than preventing all event is to simply declare the 
button type. As button without any type are treated as submit buttons.

This new class makes sense logically, does not have an effect on the 
children though.

A new variable is introduced for CompoundComponents: 
"allowClickThrough". With this set components can set their children and 
overlays to allow clicking on child elements or not. In the editor this 
is not wanted as the elements internal structure is different and the 
resulting event target can not be reliably styled. There we need the 
(span) overlay.
In the player though elements should be clickable as usual. The player 
can completely ignore this setting as it is on true (allowing clicking) 
by default.

- Remove fallback of TranslationService to allow empty error messages
  that can still cause red borders
- Remove the case distinction for checkboxes, since the requiredTrue
  validator also sends required as errorKey and a special fallback text
  is no longer used

Since CompoundElementComponent now inherits from ElementComponent,
a case distinction can no longer be made implicitly via the property
domElement.

They both have an element model and makes sense that a compound 
component is also a 'normal' component.

SHould now always correctly line up with the text even on different line 
heights.

Need to bubble the event further up to where there is a section 
reference.

Was not correctly setting the pointer event for ClozeComponents.

This uses a special event fired only by ClozeComponent, but potentially 
future compound elements as well.
The selection service gets a special method bypassing the usual 
selection logic and just firing the selection event for the props panel.

Similar to a text element but parses the text on change an replaces 
markers with actual elements.
The text is then displayed inline with the elements.

This allows to use them in template bindings. Used in compound elements.