aria-sort should be allowed on multiple columns

[jnurthen]

aria-sort states
"For each table or grid, authors should apply aria-sort to only one header at a time."

It is relatively common in large tables to allow multiple columns to be used in the sort criteria.
Sort by column 1 then sort by column 2 etc.
We should be able to use aria-sort for this.

[hartman]

See also: #582

[joanmarie]

Adding the "Agenda" label: I don't think this needs to be an ARIA 2.0 thing. Tables exist in the wild that have this functionality, and I don't believe it would be hard to support this in ATs or browsers.

[cookiecrook]

@jnurthen found an example that uses shift-click to sort multiple columns: https://datatables.net/examples/basic_init/multi_col_sort.html

I don't think this is a particularly clear user interface—Of the sorted columns, which has priority?—but since it's being used in sites, I see no issue with allowing aria-sort on multiple elements.

[cookiecrook]

Quick brainstorm: the "multi-column sort order/priority" problem might be solved as a new attribute aria-sortorder: null | 1 | 2 | n… or as a variant value on the existing attr, maybe: aria-sort="ascending second" or aria-sort="ascending 2"... Musing. Not thoroughly considered and no strong preference.