[orca-list] Experimental focus versus browse mode committed to master

[Joanmarie Diggs <jdiggs igalia com>]

Hey guys.

Based on the feedback received, I have just committed the following
changes to master:

* Orca will automatically switch between "focus mode" and "browse mode"

* Orca will announce "focus mode" or "browse mode" when it does this
  for you. It is currently at the end of the presentation of the new
  location. That way, if you know what the deal is (of course it is in
  focus mode now), you don't have to listen to Orca tell you. You can
  just keep browsing or interacting with the widget.

* The current keybinding is Orca + A. This will toggle between the two
  modes. I chose this because we needed something for you to test. And
  it was the only suggestion I saw proposed by y'all which didn't
  conflict with existing bindings or result in complaints from someone
  else. If y'all don't like Orca + A, propose something we all can live
  with or rebind it to something you can live with.

* All the non-perfomant and non-reliable logic about whether or not Orca
  should really control things is ripped out. Arrowing seems a bit more
  peppy to me now. I hope you find the same.

What I have not committed/done yet:
* Playing tones. That's a nice to have. We can add it later.
* Documentation. We don't know what the final feature will look like.
* Settings. Orca does it automatically. It's easy to toggle off.

What would be extremely helpful is testing from you to see what you all
think. What is totally broken, what must be changed, etc. In the
meantime, I'll see what I can do about the looping, etc.

Thanks!
--joanie