Implement windows 8.1+ notifications

This splits notifications up into multiple backends
currently only libnotify on unix and win8 toasts.

The win8 backend was originally written by @leeter
though heavily modified.

po/POTFILES.in

@@ -32,6 +32,7 @@ src/fe-gtk/joind.c
 src/fe-gtk/maingui.c
 src/fe-gtk/menu.c
 src/fe-gtk/notifygui.c
+src/fe-gtk/plugin-notifications.c
 src/fe-gtk/plugin-tray.c
 src/fe-gtk/plugingui.c
 src/fe-gtk/rawlog.c