[orca-list] More flat review changes: Menus in menu bars are now flat reviewable (and more!)

[Joanmarie Diggs <jdiggs igalia com>]

Hey all.

More flat review unstable goodness in master for you to play with.
Here's what's changed:

* Improve accuracy with respect to what is truly on screen
* Further improve efficiency building context (I shaved more time off
  of ginormous t-bird tables).
* Add toolkit-specific handling for special cases (and toolkit bugs)
* Make menu-bar menus reviewable, including separators and disabled
  items which cannot be arrowed to natively. (Note that other menus and
  menu-like popups are not yet being addressed, but will be in a future
  commit.)
* Fake the text interface for widgets whose implementation (or lack
  thereof) is toolkit dependent, and which typically display text.
  This is needed to review the displayed text by word and character.

I need to run an errand (that I'm wicked late for because I wanted to
finish this new batch). In the meantime, open a menu that's on the menu
bar when you're inside the menu, use flat review. More later. Have fun
testing! Looking forward to your feedback.

--joanie