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.

Improve general structure. Image situation still bad.

Since sections in dynamic mode require the "contents" display setting
the background color setting must be outsourced.
This way scroll bars are avoided.

- Images now scale up and down depending on the available space.
- The new property controls the first (column) size parameter for the 
grid. The others are fixed at 1. Decimal values are allowed for 
fine-tuned control.
This prop is passed on to the child likert-row component. It uses the 
same grid and needs this info for proper alignment with the column 
headers.
- Remove unnecessary div in likert

- List items only have a margin between then but no longer on the 
outside.
- Remove unused background color. This is set dynamically in the 
template.

This looked bad having a null value in the DOM.
16 is the default font size of the app right now.

- Reordered the buttons to make it look nicer (but still make sence 
logically)
- Put some more divs with flex layouting to prevent elements moving 
unexpextedly.
- Changed color and list inputs to split-buttons. Those compone a button 
with a dropdown.
- The editor now saves the set color and  it can be applied to 
subsequent selections as well.

ProseMirror seems not to allow editing text nodes. Since a text node can 
be a child of an element being applied the bullet list to, it fails for 
that text element. It seems it was workingly correctly before. The error 
seen on the console was inconsequential since we don't want to apply 
attributes to pure text nodes anyway. This change at least removes this 
error.

Move project detection of ElementComponent from AfterViewInit to
AfterContentChecked

The explicit check for undefined ensures that the value false of a
boolean property is taken from serialized elements.