Tree: support DnD between trees/plain containers connected to different stores

[bill]

When dragging from one Tree to another, and the Trees are connected to different stores, there are (at least) two pieces of missing functionality:

1) delete items from old store

dojo/dnd/Source.js has this code run on drop:

if(!copy && this.creator){
	source.deleteSelectedNodes();
}

There's no equivalent code for when the target is a Tree. Thus, the element won't be deleted from the source, regardless of whether the source is a Tree or not.

If the source is a Tree, it becomes more complicated, because the TreeNode being dragged may represent an item with multiple parents. In this case, should the whole item be deleted, or just unlink the parent/child relationship indicated by that TreeNode? Probably the former.

2) adding nodes to target

If the user drags "Asia" from TreeA to TreeB, seems like it should move not only "Asia", but all the descendants of "Asia" too, some of which may not even have been fetched from the data store yet.