effects/desktopgrid: Restore position correctly when dropping a window

This required a bit of a magic on the WindowHeap side to store and
restore global position of a WindowHeapDelegates' window thumbnails.

An additional property bool animationEnabled on a delegate level enables
the heap to restore position without playing unneeded initial animation,
just like the heap itself.

Windows that are being dragged or already returning form a drop are
positioned higher than others on a z-stack.

BUG: 453995