Improvement: ARGB window matching & Enable track_focus with D-Bus

- Add predefined matching target "argb" to match ARGB windows.

- Make it possible to enable focus tracking on-the-fly with D-Bus
  method.

src/common.h

@@ -1491,6 +1491,9 @@ win_set_focused_force(session_t *ps, win *w, switch_t val);
 
 void
 win_set_invert_color_force(session_t *ps, win *w, switch_t val);
+
+void
+opts_init_track_focus(session_t *ps);
 //!@}
 #endif