Support aria-atomic

[AriPerkkio]

When a node changes, the AT should look at the current element and then traverse the ancestors to find the first element with aria:atomic set.

1. If none of the ancestors have explicitly set atomic, this is the default (aria:atomic="false") case, and the AT only needs to present the changed node to the user.
2. If aria:atomic is explicitly set to false, then the AT can stop searching up the ancestor chain, and should present only the changed node to the user.
3. If aria:atomic is explicitly set to true, then the AT should present the entire subtree that aria:atomic="true" was set on.