Allow to get a route reference from SessionObject*

The motivation is to allow a Processor (here Lua) to get a pointer
to the owning Route without resorting to iterative lookup.

libs/ardour/ardour/route.h

@@ -378,6 +378,8 @@ public:
 	XMLNode& get_processor_state ();
 	virtual void set_processor_state (const XMLNode&);
 
+	boost::weak_ptr<Route> weakroute ();
+
 	int save_as_template (const std::string& path, const std::string& name);
 
 	PBD::Signal1<void,void*> SelectedChanged;

libs/ardour/route.cc

@@ -117,6 +117,11 @@ Route::Route (Session& sess, string name, PresentationInfo::Flag flag, DataType
 	processor_max_streams.reset();
 }
 
+boost::weak_ptr<Route>
+Route::weakroute () {
+	return boost::weak_ptr<Route> (shared_from_this ());
+}
+
 int
 Route::init ()
 {