- Now only UIElements are found
- HTML parts of the text editor are ignored

- remove prefix 'on'
- split methods

- Remove prefix 'on'

- Remove prefix 'on'

Elements read as classes were overwritten by the plain objects passed to 
the constructors.

This is to convey a successful operation.

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.

- Expand the list of elements zo sanitize to
['text-field', 'text-area', 'text-field-simple', 'spell-correct']
- Rename sanitize method
- Change expectations in tests

'text-field' and 'drop-list' now need '-simple' suffix.

Reloading a UnitDefinition includes loading the current UnitState

Makes more sense logically to pass the cloze element as a whole instead 
of just the document. Knowing about the document is the responsibility 
of the method not the caller.

- Fix DropLists with copyOnDrop to display a placeholder copy of
dragged elements.
The drop preview is hidden on those lists. It still has to be in the DOM
to be the target for the 'falling' animation of the dragged item,
otherwise it flies to the top left corner of the viewport.

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.

The span element created by using rich text via innerHTML makes the 
click listener not trigger. Making the innerHTML 'click through' solves 
the issue.

#244