AutomatableModelView/ContextMenu: option for unlinking all controls

Integrated patch by Rodrigo Rodrigues da Silva which adds an option
to all AutomatableModelView's context menu allowing to unlink all
linked controls.

include/AutomatableModel.h

@@ -180,6 +180,8 @@ public:
 	static void unlinkModels( AutomatableModel * _m1,
 						AutomatableModel * _m2 );
 
+	void unlinkAllModels();
+
 	virtual void saveSettings( QDomDocument & _doc,
 					QDomElement & _this,
 				const QString & _name = QString( "value" ) );
@@ -208,6 +210,11 @@ public:
 		return "0";
 	}
 
+	bool hasLinkedModels() const
+	{
+		return m_hasLinkedModels;
+	}
+
 
 public slots:
 	virtual void reset();

include/AutomatableModelView.h

@@ -92,6 +92,7 @@ public slots:
 	void execConnectionDialog();
 	void removeConnection();
 	void editSongGlobalAutomation();
+	void unlinkAllModels();
 	void removeSongGlobalAutomation();