event_pipe: added pipe_event enum and callbacks

Make the event_pipe (formerly main_notify) send/receive a set of
events, with a callback for each one.

The default event PIPE_EVENT_SIGNAL does not have a callback.  It
is still there for waking up the main thread, when it is waiting for
the player thread.

src/event_pipe.h

@@ -23,17 +23,41 @@
 
 #include <glib.h>
 
+enum pipe_event {
+	/** the default event: the main thread is waiting for somebody,
+	    and this event wakes up the main thread */
+	PIPE_EVENT_SIGNAL = 0,
+
+	/** database update was finished */
+	PIPE_EVENT_UPDATE,
+
+	/** during database update, a song was deleted */
+	PIPE_EVENT_DELETE,
+
+	/** an idle event was emitted */
+	PIPE_EVENT_IDLE,
+
+	/** must call syncPlayerAndPlaylist() */
+	PIPE_EVENT_PLAYLIST,
+
+	PIPE_EVENT_MAX
+};
+
+typedef void (*event_pipe_callback_t)(void);
+
 extern GThread *main_task;
 
 void event_pipe_init(void);
 
 void event_pipe_deinit(void);
 
+void
+event_pipe_register(enum pipe_event event, event_pipe_callback_t callback);
+
+void event_pipe_emit(enum pipe_event event);
+
 void event_pipe_signal(void);
 
 void event_pipe_wait(void);
 
-void
-main_notify_triggered(void);
-
 #endif /* MAIN_NOTIFY_H */