Add "Remove unused channels" option to FX-Mixer

This makes it easier to delete all FX-Channels that are not in use.

include/FxLine.h

@@ -73,6 +73,7 @@ private:
 private slots:
 	void renameChannel();
 	void removeChannel();
+	void removeUnusedChannels();
 	void moveChannelLeft();
 	void moveChannelRight();
 	void displayHelp();

include/FxMixerView.h

@@ -92,6 +92,9 @@ public:
 	// notify the view that an fx channel was deleted
 	void deleteChannel(int index);
 
+	// delete all unused channels
+	void deleteUnusedChannels();
+
 	// move the channel to the left or right
 	void moveChannelLeft(int index);
 	void moveChannelRight(int index);