Displaying a message is task of the host
#398

- 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

The native browser-drag-and-drop does not work in tablets. Therefore 
back to Material which works.
Features are moved over with the exception of replacing items.

- Copy-Lists now always allow putting items back.

- Renew example files
- Update versions and notes

- 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

#405

#381

#409

- Add left and right padding for input field

- Set the duration of highlighting anchors to 15 seconds
- Disable simultaneous highlighting of anchors

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

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