Partly fixed issue with updating an item's subgroup (see #745)

lib/timeline/component/ItemSet.js

@@ -1027,12 +1027,13 @@ ItemSet.prototype._addItem = function(item) {
  */
 ItemSet.prototype._updateItem = function(item, itemData) {
   var oldGroupId = item.data.group;
+  var oldSubGroupId = item.data.subgroup;
 
   // update the items data (will redraw the item when displayed)
   item.setData(itemData);
 
   // update group
-  if (oldGroupId != item.data.group) {
+  if (oldGroupId != item.data.group || oldSubGroupId != item.data.subgroup) {
     var oldGroup = this.groups[oldGroupId];
     if (oldGroup) oldGroup.remove(item);