Exclude inactive tracks & busses from stem export list

The exported file is silent, when choosing to export either region contents or
track outputs, so there seems no point in making such routes available for stem
export at all.

Perhaps there's an arguable use case for enabling stem export of inactive
tracks, but I can't see that it's worth worrying about for now.

gtk2_ardour/export_channel_selector.cc

@@ -647,12 +647,21 @@ TrackExportChannelSelector::fill_list()
 			if ((*it)->is_master () || (*it)->is_monitor ()) {
 				continue;
 			}
+			if (!(*it)->active ()) {
+				// don't include inactive busses
+				continue;
+			}
+
 			// not monitor or master bus
 			add_track (*it);
 		}
 	}
 	for (RouteList::iterator it = routes.begin(); it != routes.end(); ++it) {
 		if (boost::dynamic_pointer_cast<AudioTrack>(*it)) {
+			if (!(*it)->active ()) {
+				// don't include inactive tracks
+				continue;
+			}
 			add_track (*it);
 		}
 	}