Add the NestedSort extension: stable sorting for Grid instances.

[dmartin-gh]

Nested grid sorting was briefly mentioned in this pull request. This was a feature we needed internally so we've gone ahead and developed the extension.

The dgrid/extensions/NestedSort module provides stable sorting for Grid instances. It overrides the _constructSort() method, which is in charge of taking a targeted column that has presumably been clicked on and returning an array to set as the active sort. I've included intern tests for the extension.

Since this is my first non-trivial request, I've submitted an Individual CLA to The Dojo Foundation. A Corporate CLA for RGM Advisors LLC will be submitted soon to cover my individual one.

Please let me know if you would like anything else added to the branch.

[dmartin-gh]

Haven't seen any traffic on this one. Is this planned on being implemented elsewhere or is stable sorting not on the table? Our CLAs have been active for a while now,

[dylans]

hi @zarillion We would like to look at it eventually, but it may be a while before we can prioritize it above many of the other things we're trying to do to improve dgrid. We definitely appreciate your contributions, efforts, and patience.