This adds a null notification backend implementation for win32, purely
to avoid crashes due to a missing backend when applications use
GNotification. This backend does nothing except print a warning when a
notification is supposed to be emitted.
In future, it can be expanded to use win32 API to present toaster
notifications appropriately.
Signed-off-by: Philip Withnall <withnall@endlessm.com>
https://bugzilla.gnome.org/show_bug.cgi?id=776583
