- Add ReferenceManager to check if deleted elements are referenced 
anywhere in the unit. If so display a warning.
- Move page logic to UnitService

Remove 'any's be adding proper types and using 'never'.

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.

Show accordion with expansion-panels instead of tabs.
This way elements can be grouped logically.

Components with bigger scope are put to the component root, canvas, 
props panle and new-element-panel.

- Move value input to dropList input group

With this setting elements are copied when being dropped to another 
list.

On drop it is also checked if the item-ID is already present in the 
list. If it is the drop event is silently discarded. This allows putting 
items back in the list without creating duplicate IDs.

Lists with this setting:
- do show a placeholder of the items being dragged. This way it is 
conveyed that the item will remain there after being dropped.
- don't show a placeholder when items are hovered over them, to avoid 
confusion with duplicate items.

Sorting within DropLists is disabled, since it causes moving items 
within the list. It has to be discussed if this feature is even needed.

- Split text field and drop list, so they can have differing props
- Improve their dimension default values
- Improve alignment of all cloze children components
- Add dynamic width parameter of toggle buttons to dimension properties
panel. Therefore the postion tab can always be shown and no longer needs
a condition.

Remove unused class.

- Add fieldset to make it look nicer.
- Improve image to have fixed size, scale properly and show buttons only 
on hover.
- Improve translations
- Add tooltip to actionparam dropdown: When having selected pageNav it 
shows a tooltip to get across that only valid pages can show up.

#240

since the player uses it as well

Change import paths of CommonModule files in editor and player

The solution is to always show the section menu of the selected section. 
No more hovering shenanigans.

#223

verticalButtonAlignment is the new property for likert rows, which tells 
the actual radio button to either be at the top or in the middle.

Also rename LikertColumn to ColumnHeader, since it is not exclusively 
used by likert.

The LikertColumn is actually just an image-text-combo. Has a generalized 
name now.
The other interface was not needed anymore and has been deleted.

- A new new editor component has been created for this, which has a
reduced interface and produces span-elements instead of paragraphs.
- Since the created HTML (span with CSS) is regarded as potentially
harmful it needs to be sanitized. For element components the sanitizer
is put in the abstract parent element-component-directive.

This makes for smaller more managable components.

This allows showing the contained text in multiple columns.

The text area was mistakenly shown.

There were several issues with it's properties, which could not be 
changed.
Now it is properly initialized and prop change get actually handled.

This helps identifying the selected element.