Dijit: data binding smarts and properties need to be a mixin

[alex]

Currently tree and the select/autocomplete define properties for locating stores are implemented on an ad-hoc basis. Properties like query, labelAttr, typeAttr, and store should be exposed through a general mixin that knows how to marshall those properties and, at a minimum, locate stores referenced by name.

This is probably just extracting code from Tree.

[bill]

See also #3025

[bill]

As it turns out, "labelAttr" and "typeAttr" have been removed, "query" is only for Tree, and mapping a store name to the store itself is done by the parser. So if there was a base class it would just be one line:

store: null

Marking as "won't fix" for now; if things change later we can make a mixin.