Make MIDI busses possible targets of internal sends

The session only added an internal return to new audio busses, and so
only those were proposed in the "New Aux Send" list. Also add the return
to new midi busses, now that internal sends know how to deal with midi.

libs/ardour/session.cc

@@ -2580,6 +2580,7 @@ Session::new_midi_route (RouteGroup* route_group, uint32_t how_many, string name
 				route_group->add (bus);
 			}
 
+			bus->add_internal_return ();
 			ret.push_back (bus);
 		}