Ref T1344. Allows you to drag tasks within a column and between columns, and handles all the multi-column state / targeting / ghosting stuff.
This is a UI-only change; you can't actually do anything meaningful with these yet.
Roughly, I added the idea of a DraggableList existing within a "group" of draggable lists. Normally, that group only has one item, but on boards it has all of the columns. Then I made all of the relevant operations just apply to the whole group of lists.