enable dnd under a tree node if the node is expanded

[moromete]

targetRow = targetRow.nextSibling;

nextSibling is not necessary a grid row. It can be a "dgrid-tree-container" so we must skip it until we find next tree row.

[moromete]

with this override of the _getChildByEvent method of dojo/dnd/Container
https://gist.github.com/moromete/5263027

and with this commit https://github.com/moromete/dgrid/commit/d8f3cc80e38a96c3dd47e3d5c209d63c5bbc8d78
we can dnd any node at any level of the tree grid (dgrid + tree extension) providing also the "after" information