i3bar: set _NET_SYSTEM_TRAY_COLORS for symbolic icons (gtk3+)

i3bar/include/xcb.h

@@ -88,6 +88,15 @@ void get_atoms(void);
  */
 void kick_tray_clients(i3_output *output);
 
+/*
+ * We need to set the _NET_SYSTEM_TRAY_COLORS atom on the tray selection window
+ * to make GTK+ 3 applets with Symbolic Icons visible. If the colors are unset,
+ * they assume a light background.
+ * See also https://bugzilla.gnome.org/show_bug.cgi?id=679591
+ *
+ */
+void init_tray_colors(void);
+
 /*
  * Destroy the bar of the specified output
  *