Drag.hotSpot needs width and height too

Currently the Drag.hotSpot attached property has only x any y.
It needs width and height properties too. This is needed in order
to support use cases such as when e.g. a "toolbox" UI element is
docked e.g. at the top or side of a window. The docking should take
effect when the toolbox touch the top or side of the window, and
for this to work a single-point hotSpot is not enough.