R1:5dca1569b577

Preview the effects of a drag-and-drop operation on workboards

Summary:
Ref T10335. Ref T5474. When you drag-and-drop a card on a workboard, show a UI hint which lists all the things that the operation will do.

This shows: column moves; changes because of dragging a card to a different header; and changes which will be caused by triggers.

Not implemented here:

- Actions are currently shown even if they have no effect. For example, if you drag a "Normal" task to a different column, it says "Change…