Add group role and aria-labeledby attribute to groups.

[tlfdev]

I am a native screen reader user. The groups example on the start page do not present the groups to the screen reader. Because groups are essentially div elements, they do not have the proper role, nor do they have the proper label.
In this fix, I assigned an ID if no ID existed to the group heading label. Then I used that ID to populate the aria-labeledby attribute. The secondary fix was to add a role of group to the div, so that screen readers and the accessibility tree know that the div is a group.
Now when navigating through this combobox, I hear the group heading as I arrow to an item in that group with the number of items in the group. This is proper and expected behavior.

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