Add vtables for DND
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.
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