Improve integration with dijit/Tree's DnD

[dylans]

When attempting to integrate dijit/Tree (including DnD), and keeping the Tree and dstore instance synchronized (similar to the example in https://dojotoolkit.org/reference-guide/1.10/dijit/Tree.html#updating-a-tree ), we received a report that this was not possible to get working with the dijit/Tree's ObjectStoreModel. It was reported that the update functionality (Observer/Trackable) did not seem to work out of the box with the Tree. This also does not work with dojo/store/Memory because of https://bugs.dojotoolkit.org/ticket/15660 )