message: Add asset add / remove messaging clauses.
diff: @@ -624,6 +624,12 @@ const onMessage = function(event) {
  } else if (event.data.action === "set_widget") {
  setWidget(event, roomId);
  return;
  + } else if (event.data.action === "add_widget_asset") {
  + addWidgetAsset(event, roomId);
  + return;
  + } else if (event.data.action === "remove_widget_asset") {
  + removeWidgetAsset(event, roomId);
  + return;
  }
  if (!roomId) {
  