Move label creation to ElementFactory.

No way has been found to have an abstract static method for 
OptionElement's label generation, therefore silence the linter.
Same for getElementComponent.

This is only useful in CompoundElements. So use checks for that class 
instance.

So TipTap does not moan about empty document.
Put placeholder text in the cloze document instead of the component 
template.

Now checks when creating or updating object if IDs are okay.

Now always shows the values as rich text (which works because basic text 
is valid html).

Also remove some unused fields.

A no longer existing class was used, but turned out to not be necessary 
anyway.

#313

Initialize to null, to be able to differentiate in the props panel.

Also improve props panel layout by always showing all fields and 
disabling them if needed instead of hiding them.

Fix translation.

#318

sanatizer
TextImageLabel:
 position -> imgPosition

DragNDropValueObject
 stringValue -> text
 imgSrcValue -> imgSrc

RadioButtonGroupComplexElement
 columns -> options

Likert
 columns -> options

#293

Also add it to the likert row dialog.

#112

#283

Also refactor properties panel and ToggleButton class logic for striking 
values. This now relies more on CSS classes provided by Material instead 
of doing finetuned logic my ourselves. This also allows showing the 
correct preview in the editor.

#287

- Move props to separate files
- Fix typing

Also improve media element property control layout.

#204

Now the dialog scrolls and the buttons at the bottom can be reached even 
on low viewport height.

#294

#283

Casting to ButtonElement caused issues where changed values were not 
recognized by inputs.

#280

The TipTap editor needs a setting to be able to read base64 encoded 
images.

Also improve method name.

#297

#298

..instead of multiple methods doing almost the same stuff.