dnd: Show an X pointer when dragging over an invalid area.

Some additional callbacks could be implemented for DnD target
locations, as well as some sort of "Stage" handleDragOver function.
Some possibilities include, but are not limited to,
 - Allowing drag and drop onto the desktop (nemo - launchers, files?)
 - Allowing dragging of applets onto the stage to remove them (a la browsers)
 - A little more intelligence in cursor types and when/where to use them.
   I think we need some sort of trash/recycle cursor.  We can't use the X
   for both 'invalid target' and 'erase this' intents.