remove GUI object state when tracks/busses are removed.

Not doing this results in accumulation of useless GUI object state which is never cleaned up.
Taken from work by Grygorii Zharun.

gtk2_ardour/automation_time_axis.cc

@@ -285,6 +285,7 @@ AutomationTimeAxisView::AutomationTimeAxisView (
 
 AutomationTimeAxisView::~AutomationTimeAxisView ()
 {
+	cleanup_gui_properties ();
 	delete _view;
 }
 

gtk2_ardour/axis_view.h

@@ -70,6 +70,12 @@ class AxisView : public virtual Selectable, public PBD::ScopedConnectionList, pu
 		property_hashtable.emplace(property_name, s.str());
 		gui_object_state().set_property<T> (state_id(), property_name, value);
 	}
+    
+	void cleanup_gui_properties () {
+		/* remove related property node from the GUI state */
+		gui_object_state().remove_node (state_id());
+		property_hashtable.clear ();
+	}
 
 	bool marked_for_display () const;
 	virtual bool set_marked_for_display (bool);

gtk2_ardour/gui_object.cc

@@ -71,6 +71,16 @@ GUIObjectState::get_or_add_node (const string& id)
 	return get_or_add_node (&_state, id);
 }
 
+/** Remove node with provided id.
+ *  @param id property of Object node to look for.
+ */
+
+void
+GUIObjectState::remove_node (const std::string& id)
+{
+	_state.remove_nodes_and_delete(X_("id"), id );
+}
+
 /** Get a string from our state.
  *  @param id property of Object node to look for.
  *  @param prop_name name of the Object property to return.

gtk2_ardour/gui_object.h

@@ -47,12 +47,14 @@ public:
 		s << val;
 		child->add_property (prop_name.c_str(), s.str());
 	}
-
+    
 	std::list<std::string> all_ids () const;
 
 	static XMLNode* get_node (const XMLNode *, const std::string &);
 	XMLNode* get_or_add_node (const std::string &);
 	static XMLNode* get_or_add_node (XMLNode *, const std::string &);
+    
+	void remove_node (const std::string& id);
 	
   private:
 	XMLNode _state;

gtk2_ardour/route_time_axis.cc

@@ -323,6 +323,8 @@ RouteTimeAxisView::set_route (boost::shared_ptr<Route> rt)
 
 RouteTimeAxisView::~RouteTimeAxisView ()
 {
+	cleanup_gui_properties ();
+	
 	for (list<ProcessorAutomationInfo*>::iterator i = processor_automation.begin(); i != processor_automation.end(); ++i) {
 		delete *i;
 	}

gtk2_ardour/route_ui.cc

@@ -87,9 +87,11 @@ RouteUI::RouteUI (ARDOUR::Session* sess)
 
 RouteUI::~RouteUI()
 {
+	gui_object_state().remove_node(route_state_id());
+
 	_route.reset (); /* drop reference to route, so that it can be cleaned up */
 	route_connections.drop_connections ();
-
+    
 	delete solo_menu;
 	delete mute_menu;
 	delete sends_menu;