- Also simplify the button constructor

- A HotspotImage has an image and divs placed on it, which are used
as hotspots
- Inherits from FormElementComponent, which is why the hotspots are kept
  as a list in the value property.
- Can be validated as required and uses its own custom validator

- Fix element creation to have new value objects instead of referencing 
the old one
- Add new IDs to DropList value objects

Options are now possibly rich text. This needs to be taken care of.
Also use extra span tag to preserve all whitesoaces in the index-suffix.

#279

- add translations

- Make position props on buttons optional
- Move UIElement creation back to Factory instead of within Section

#294

- remove ID handling from model classes. This is now done by the editor 
itself (mainly unit service).
- Rename and move IDManager to editor as IDService

Just called from the editor, before creating the Unit instance.
It goes through all IDs and checks for duplicates. If found it generates 
a new one and shows the prompt.

- Do ID checks only in the class, not in unit service (or elsewhere)
- Fix repaired value list to get new objects instead of references to 
the existing.

Aslo create new props panel component and move border stylings to 
separate fieldset (wihtin the styling tab).

#291

One as a general label above the table and one for the first column 
heading.

Since this is no proper InputElement separate properties panel inputs 
are used.

#272

Add new parameter 'itemsPerRow'. This limits the grid columns used, 
making items move to the next row if overextending.

Also simplify component structure.

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

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

Also remove some unused fields.

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.

Also add it to the likert row dialog.

#112

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