Tree: finalize design for specifying root node

[bill]

Before beta, need to finalize design for specifying root node of tree.

As per #5812, the current root node specification is polymorphic:

new dijit.Tree({ root: 5, ... });
new dijit.Tree({ root: "continents", ... });

Unfortunately this doesn't fly well with the parser.... root is always treated as a string.

<div dojoType="dijit.Tree" root="5">

... even if it looks like a number

[bill]

(In [12481]) Specify root item of tree using query parameter (like in v1.0) rather than "root" parameter. But query returning multiple items (rather than a single item) is deprecated; plan to remove in 2.0. Fixes #5879. !strict