Tree Pattern: Update selection guidance to include aria-checked

[mcking65]

ARIA 1.3 clarifies when browsers may provide implicit selection.
Those changes to the ARIA specification enable APG to provide guidance on using ariachecked to represent selection in trees. This change updates the roles, states, and properties section of the tree view pattern accordingly.

Preview changes to tree pattern in RawGitHack

Note: revisions to listbox will be addressed separately.

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[JAWS-test]

I haven't followed the entire discussion on the topic, but it seems odd to me that in the paragraph aria-checked and aria-selected can be used arbitrarily. Shouldn't there be some indication that primarily aria-selected should be used. Only if selected and checked are two different states that a treeitem has, both should be used?

[mcking65]

@JAWS-test wrote:

I haven't followed the entire discussion on the topic, but it seems odd to me that in the paragraph aria-checked and aria-selected can be used arbitrarily. Shouldn't there be some indication that primarily aria-selected should be used.

I'm planning to add a note related to the choice of selected or checked before this is ready for review.

Only if selected and checked are two different states that a treeitem has, both should be used?

In the spec, We included a set of 3 conditions that would have to be met by a tree that uses both checked and selected. Basically, we strongly advise against designing UI that would use both. It would be very difficult to ever meet all three conditions. I'm also planning to add a note about this as well.