Add platform mappings for role-change notifications

[joanmarie]

Contrary to what the ARIA spec used to suggest, platform accessibility APIs may indeed have a vehicle to notify assistive technologies that a role has changed. For instance, in ATK there are property-change signals. And one of the properties which can change happens to be accessible-role.

Core AAM already has a bunch of tables outlining platform-specific events/notifications which should be emitted when something changes. For some reason, role isn't there. It should be.

[joanmarie]

BTW, the deleted text included the following normative statement:

Since assistive technologies might not detect that the role has changed, user agents MAY address this condition by removing the item from the accessibility tree and inserting a new item in its place.

We should probably preserve that in the new role-change content we're going to create.