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

Also remove debug output and improve formatting.

- Add originListID and originListIndex to all DragNDropValueObjects

- 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.

- Add pipe to decide which list to use

- Add pipe to decide which list to use

- Add 'Button' as possible child of cloze elements

The property vertical-align is interpreted differently by Chrome based
browsers, so a special webkit value is needed to override the value. The
browser specific value is used last, so it is used by browsers which can
interpret it.
#414

- text field simple
- toggle button

(The array reference must not be destroyed)

#405

#381

#409

- Add cdk scrolling strategy to keyboard that brings overlays to
scrolling

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

- Add left and right padding for input field

- When the height of the child elements of a cloze element is increased,
they all now grow out of their center

Read-only math fields are displayed without borders

HTML labels can cause errors in the Test Center for units in XML format.