Select the clicked route in the current mixer strip (fixes #7429)

Issue #7429 reports that that clicking a route of an already selected group
does not update the editor mixer strip selection. To fix this we call
Editor::set_selected_mixer_strip() at the end of
RouteTimeAxisView::selection_click();

The overhead of maybe calling it twice should be toleratable, as
::set_selected_mixer_strip() checks, if the route is already the current mixer
strip route before setting it.

gtk2_ardour/route_time_axis.cc

@@ -1340,6 +1340,8 @@ RouteTimeAxisView::selection_click (GdkEventButton* ev)
 	}
 
 	_editor.commit_reversible_selection_op ();
+
+	_editor.set_selected_mixer_strip (*this);
 }
 
 void