Explicitly suspend redisplay while adding/removing tracks

This should not be needed, however Editor::idle_remove_tracks()
has the same priority as Editor::redisplay_track_views() and this
might save us another redisplay call.

gtk2_ardour/editor.cc

@@ -5669,6 +5669,8 @@ Editor::add_stripables (StripableList& sl)
 
 	sl.sort (Stripable::Sorter());
 
+	DisplaySuspender ds;
+
 	for (StripableList::iterator s = sl.begin(); s != sl.end(); ++s) {
 
 		if ((*s)->is_foldbackbus()) {
@@ -5747,6 +5749,8 @@ Editor::timeaxisview_deleted (TimeAxisView *tv)
 		return;
 	}
 
+	DisplaySuspender ds;
+
 	ENSURE_GUI_THREAD (*this, &Editor::timeaxisview_deleted, tv);
 
 	if (dynamic_cast<AutomationTimeAxisView*> (tv)) {