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

Turns out the space within the template code are doing this.

#270

Italics indicate that this is not a proper text.

This is needed in on drop logic to set the new value.

- remove prefix 'on'
- split methods

This way elements can handle their logic themselves without having to
rely on outside utility classes.

Also restructure files in common in a more logical way.