Massive changes to OLE2 DnD protocol, which was completely broken before:
* Keep GdkDragContext and OLE2 objects separate (don't ref/unref them
  together, don't necessarily create them together).
* Keep IDataObject formats in the object itself, not in a global variable.
* Fix getdata() to look up the request target in its format list, not in the
  global hash table
* Create target GdkDragContext on each drag_enter, destroy it on drag_leave,
  whereas IDropTarget is created when a window becomes a drag destination
  and is re-used indefinitely.
* Query the source IDataObject for its supported types, cache them in the
  target (!) context. This is how GTK+ works, honestly.
* Remember current_src_object when we initiate a drag, to be able
  to detect later on that the data object is ours and use a
  shortcut when querying targets
* Make sure GDK_DRAG_MOTION is only sent when something changes
* Support GTK drag cursors
* Ensure that exotic GTK clipboard formats are registered
  (but...