This commit hides GdkDragContext and GdkDragContextClass, adds
vfuncs for most drag context functionality, and turns the X11 DND
implementation into GdkDragContextX11. We also add vfuncs to
GdkDisplay for gdk_drag_get_protocol and to GdkWindow for
gdk_drag_begin, and implemenet them for X11.
Other backends need similar treatment and are broken now.