#521

#521

- Replace or remove no longer used material customization

#521

- Remove no longer used material customization

Also restructure files containing interfaces.

- Reformat styles

- Reformat styles

- Reformat styles

- Reformat styles

- Replace UpdateTextareaPipe with DynamicRowsDirective

The dynamic height of the textarea is calculated based on the available
width. However, accessing the width of the textarea often results in an
error.
For this reason, the calculation is now done using resize and change
events in conjunction with a timeout to wait for the textarea to render.

- 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