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 priority to Normal.". I plan to hide actions which have no effect, but figuring this out is a little bit tricky.
• I'd like to make "trigger effects" vs "non-trigger effects" a little more clear in the future, probably.

Dragged stuff between columns and headers, and into columns with triggers. Got appropriate preview text hints previewing what the action would do in the UI.

(This is tricky to take a screenshot of since it only shows up while the mouse cursor is down.)