Remove flowTo alias on accessibleNode

[cookiecrook]

trafficstars

The flowTo property on accessibleNode is based on aria-flowto which has ambiguities (looping, reversing) we should not repeat in AOM. It also would require disambiguation with the node ordering planned for virtual nodes in Phase 3.

Authors wanting to use aria-flowto can still implement it via DOM methods like setAttribute, so we're not losing anything by not including the alias on accessibleNode.

(Spawned from discussion in Issue #78)

[cookiecrook]

Proposed deprecation of aria-flowto in ARIA Issue #630

[minorninth]

I fully support deleting it from the AOM spec if it's deprecated in ARIA.

Otherwise I'd strongly prefer we just map all non-deprecated ARIA attributes to AOM rather than trying to pick favorites.

[brennanyoung]

aria-flowto is not (yet) deprecated, and addresses a use case that is not met (or planned to be met) in any other way.