win32: get rid of GdkDragContextPrivateWin32 and related machinery.

win32: get rid of GdkDragContextPrivateWin32 and related machinery.

This turns GdkWin32DragContext into a proper GdkDragContext subclass.
Because we now correctly initialize GdkWin32DragContext in
gdk_drag_context_new, we no longer crash immediatly when a DnD
operation is initialized (the find_window, drag_status, ... vfuncs
where all pointing to 0x0 instead of their proper win32 implementations).

We now try to consistently refer to GdkDragContext as "context",
GdkWin32DragContext as "win32_context" and the ole2 related
target_drag_context and source_drag_context as "ctx".

Members of GdkWin32DragContext only used by the ole2 DnD codepaths
are now explicitly marked with a ole2_dnd_ prefix.