Properly forward MutePointChanged to mutable

The GUI only subscribes to the mutable's mute_points_changed
signal, without direct access to to the MuteMaster itself.

Previously changing mute points by means other than the GUI,
did not update existing context menus in the mixer GUI.

libs/ardour/muteable.cc

@@ -24,4 +24,5 @@ using namespace ARDOUR;
 Muteable::Muteable (Session& s, std::string const & name)
 	: _mute_master (new MuteMaster (s, *this, name))
 {
+	_mute_master->MutePointChanged.connect_same_thread (*_mute_master, [&] { mute_points_changed (); });
 }