extract a dijit._DataBound mixin, refactor data-bound widgets to use it

[alex]

currently, many widgets have some common data-binding properties and events. These should be extracted and codified. Namely, the following seem important for a first cut:

{
  store: null,
  query: null,
  pageSize: 100,
  ...
}

[jaredj]

Not enough time remaining in 1.2 to make a sweeping dijit change. Therefore moving this out.