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.

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

Development

(The array reference must not be destroyed)

#405

#381

#409

Darken the border form fields, toggle buttons & simple text fields

Feature/highlight drop