i3bar: kick tray clients after output configuration changed

This makes i3bar reflect xrandr --output foo --primary changes immediately.

i3bar/include/xcb.h

@@ -77,6 +77,17 @@ void clean_xcb();
  */
 void get_atoms();
 
+/*
+ * Reparents all tray clients of the specified output to the root window. This
+ * is either used when shutting down, when an output appears (xrandr --output
+ * VGA1 --off) or when the primary output changes.
+ *
+ * Applications using the tray will start the protocol from the beginning again
+ * afterwards.
+ *
+ */
+void kick_tray_clients(i3_output *output);
+
 /*
  * Destroy the bar of the specified output
  *