add config options for communication mode with mpd (polling for default)

idle support in ncmpcpp is marked experimental since its support
in mpd is not ready for general use in full featured clients
(see bug 2612)

src/settings.h

@@ -176,6 +176,7 @@ struct ncmpcpp_config
 	
 	mpd_tag_type media_lib_primary_tag;
 	
+	bool enable_idle_notifications;
 	bool colors_enabled;
 	bool fancy_scrolling;
 	bool playlist_show_remaining_time;