effects/desktopgrid: Fix cursor shape being lost while dragging a desktop

DragHandler does not travel along with its target heap, so it only
affects the cursor shape when the mouse pointer is directly over it; but
when a desktop is dragged far outside, the cursor resets.

Besides, this is how we already implemented it in WindowHeapDelegate.

FIXED-IN: 5.26
(cherry picked from commit 9889e0df)