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.

src/common/fe.h

@@ -178,7 +178,6 @@ typedef enum
 } feicon;
 void fe_tray_set_icon (feicon icon);
 void fe_tray_set_tooltip (const char *text);
-void fe_tray_set_balloon (const char *title, const char *text);
 void fe_open_chan_list (server *serv, char *filter, int do_refresh);
 const char *fe_get_default_font ();