Mir doesn't support drag and drop (DND) windows

Mir doesn't support drag and drop (DND) windows.

In playing with X apps under Xmir, one of the more broken features is drag and drop (_NET_WM_WINDOW_TYPE_DND). Mir doesn't understand it yet, so at best you get a new window appear for the thing getting dragged, but it doesn't move.

Test cases:
  In Xmir run nautilus and drag an icon from it.
  In Xmir run chromium-browser and drag a browser tab from it.

Although X historically implements these things with absolute placement (which Mir doesn't support), we could implement DND movement in Mir shells using some simple heuristic:
   relative_grab_pos = cursor pos in the original app window when the DND window was created
   Now Mir always places the DND window at the cursor pos minus relative_grab_pos.
   Thus the surface being dragged always retains the same position under the cursor as when it was grabbed.