introduce priority support

add new command /priority <int>
improve jabber_update_presence()
use jabber_update_presence() for sending initial presence
save priority and status string to jabber_conn structure

src/windows.c

@@ -914,6 +914,8 @@ cons_prefs(void)
         cons_show("Reminder notification period : %d seconds", remind_period);
     }
 
+    cons_show("Priority                     : %d", prefs_get_priority());
+
     cons_show("");
 
     if (_curr_prof_win == 0)