gio: Add dummy win32 notification backend

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

gio/meson.build

@@ -440,6 +440,7 @@ else
     'gwin32outputstream.c',
     'gwin32networkmonitor.c',
     'gwin32networkmonitor.h',
+    'gwin32notificationbackend.c',
   )
 
   gio_win_rc = configure_file(