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.

- 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

- Mainly highlight border and moving elements.
- Centerpiece is: line-height: 130%;
  Don't know why or how but it works okay.

Rules for when drop is allowed have been fixed. It is quite complicated 
and therefore a lot of explanatory comments are added.

- improved translation
- add example file

Allows DropListvalueObjects which have this setting to be replaced in
lists which only allow onlyOneItem. When this switch is set, the class
sets the other 2 internal values
(originListID?: string; originListIndex) itself. Those can not be seen
nor changed from outside.

Also refactoring in lots of places around droplists. Better names, 
cleaner structure, comments.

#305

Lists with this setting delete dropped value objects. This allows 
putting items back.
(Only makes sense when used with copyOnDrop. Otherwise there can not be 
duplicate value IDs).

Also improve constructor value checks to correctly handle nullish 
booleans.

#281

#309

Otherwise, no elements can be dropped at this place

To detect if a custom form element was touched it is necessary to set a
custom tab index

Use the source list placeholderIndex instead of the index for the target 
list.

Chromium does not allow DOM manipulation on dragstart. Therefore add 
timeout.

- Replace Material Droplist with native HTML events
- Remove simple-drop-list element; Cloze elements now use the normal 
DropList element
- Add example units

- overflow of lists is now hidden to prevent dropable elements from
being displayed outside the container
- placeholders are hidden with flex orientation

Use a horizontal aligned placeholder for dynamic aligned drop lists
when there is more than one element in the list

- Refactor variable of the placeholder dimensions