- Remove css margins of floating keypad

- A floating keypad is generated inside a div with the class
  'cdk-overlay-pane'. Now the position of this div is updated when
  dragging the keypad.

- 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

Also unit defintion version to 3.10.0

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.

For Droplists this needs to be done when multiple items are allowed.

For ToggleButtons it needs to be set when vertical orientation is used.

It is then centered without caring about text alignment. With multiple 
item it would not be possible to align all of them anyway.

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