- Add spans with 'nbsp;' to images to be able to align drop lists of
  cloze elements with images to the baseline.

- This prevents the cursor to be set to the upper left corner of the
  preview image

- Allow put back
- check if to replace item can be moved elsewhere. this is only possible 
if it is not in it's origin.

Now checks again if the source list was a copy list and discards the 
item if so.

Hold the replaced item while doing the actual transfer and add it to 
it's origin afterward. This way it can not be transfered after being 
moved back.

- list-item is styled as placeholder
- placeholder has no dimensions

If the placeholder or the item is hidden
the other one leaves the list when dragged

Floating items can be aligned vertically and horizontally at the same 
time, but Material-DnD can't sort in both directions at the same time, 
therefore disable in this case.

- Use placeholder with invisible character to set the baseline for the
empty drop list.

Also remove debug output and improve formatting.

- Hide error message in error border while highlight receiving
  drop list border

- allowReplacement (moved from element to list prop)
- originList and Index are non-optional (means placeholders have to be 
used sometimes)
- Re-introduced the dict of all DropListComponents. This is the only way 
I have found to get a reference to a DropListComponent by ID. This is 
needed for returning to origin. Origin is not actually part of the 
transaction and needs to be fetched from somewhere else. Hence the dict.

- Console statements still present for debugging.

The native browser-drag-and-drop does not work in tablets. Therefore 
back to Material which works.
Features are moved over with the exception of replacing items.

- Copy-Lists now always allow putting items back.

- Renew example files
- Update versions and notes

- Add pipe to decide which list to use

- Add pipe to decide which list to use

#405

#381

#409

Drop lists that allow only one element but are not used in cloze texts
display their elements right-aligned