- Remove unused variable
- Add typing

- Part of the properties panel for every element (like delete and 
duplicate)
- This allows interacting with elements, generating events and inpect 
their usage. Usually this is unwanted since it might interfere with 
dragging the overlay, but sometimes you want to get a feel for the 
element or check it's contents (GeoGebra for example)

Also set geometry element to always use fixed size, which is all that 
works anyway.

- 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

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

- 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

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.

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

- Move props to separate files
- Fix typing

Also improve media element property control layout.

#204

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

#280

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