This facilitates the testability

- Reduce amount of properties
- Split code in separate methods
- Define some properties as input properties

This eliminates the need to specify the different page types in
AppComponent

Knowledge of the 'document' existence is only a matter of the function 
not the caller.

- Add fieldset and reorder fields
- Add dynamic connectedDropLists menu

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

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.

- Move value input to dropList input group

- 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.

This marks the end of the test according to the expect method

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.