Don't unset CueEditor:_track when changing regions.

CueEditor::catch_pending_show_region() calls ::set_region
which calls CueEditor::unset(false). This reset _track.

Also PianorollWindow::set() calls set_track() before set_region(),
the latter also reset CueEditor::_track.

This fixes the Solo button in the Pianoroll window, and
probably some other things, too.

gtk2_ardour/cue_editor.cc

@@ -1471,11 +1471,14 @@ CueEditor::unset (bool trigger_too)
 	count_in_connection.disconnect ();
 	capture_connections.drop_connections ();
 
-	_track.reset ();
 	_region.reset ();
 
 	if (trigger_too) {
 		ref = TriggerReference ();
+		_track.reset ();
+	} else if (_track) {
+		/* re-subscribe to object_connections */
+		set_track (_track);
 	}
 }