GNetworkMonitorWindows: Add IPv4/IPv6 network monitor backend for windows

Added a Windows backend to GNetworkMonitor, using NotifyRouteChange2()
(available on Vista and later). It marshals the route change callbacks
to the thread-specific default main context the GNetworkMonitor was
constructed in.

https://bugzilla.gnome.org/show_bug.cgi?id=685442

gio/meson.build

@@ -388,6 +388,8 @@ else
     'gwin32volumemonitor.c',
     'gwin32inputstream.c',
     'gwin32outputstream.c',
+    'gnetworkmonitorwindows.c',
+    'gnetworkmonitorwindows.h',
   )
 
   gio_win_rc = configure_file(