Clean up triggers awaiting global state modification at shutdown

Otherwise they can be reported as memory leaks since no more global
state modifications will take place to notify the trigger to clean
itself up.

src/Notifier.h

@@ -30,6 +30,12 @@ public:
 	 * @param m object that was modified
 	 */
 	virtual void Modified(Modifiable* m) = 0;
+
+	/**
+	 * Callback executed when notification registry is terminating and
+	 * no further modifications can possibly occur.
+	 */
+	virtual void Terminate() { }
 };
 
 /** Singleton class tracking all notification requests globally. */
@@ -69,6 +75,12 @@ public:
 	 */
 	void Unregister(Modifiable* m);
 
+	/**
+	 * Notifies all receivers that no further modifications will occur
+	 * as the registry is shutting down.
+	 */
+	void Terminate();
+
 private:
 	friend class Modifiable;