[orca-list] Web page summary can now be enabled on page load, includes landmarks

[Joanmarie Diggs <jdiggs igalia com>]

Hey all.

As you may know, a detailed where am I in a web page causes Orca to
present a summary of the page, such as the number of headings and links.
I have just committed a change to master so that you can opt to have
this presented automatically upon page load. From my testing so far, it
seems pretty performant so it is currently enabled by default for
Firefox. While I was doing this, I also added ARIA landmarks to the list
of things included in the summary.

Please test and let me know what you think. Thanks!
--joanie