[orca-list] Some major fixes committed to master, please test

[Joanmarie Diggs <jdiggs igalia com>]

Hey all.

Over the past couple of days, I have committed a handful of (what I
think are) major fixes. I have tested the heck out of them, but some
additional testing would be appreciated.

Performance related:

* Consider object:active-descendant-changed events as candidates of
  an "event flood." Should help in GtkTreeView instances.

* Only perform braille functions when braille is enabled. For those
  of you using speech output only, you should notice Orca being
  snappier. I found this especially true when typing in a text editor.
  (Optimizing all generation, including braille, is on my to-do list)

Other fixes:

* Work around an issue in presenting text-changed events resulting from
  BackSpace and Delete no longer claiming to be BackSpace and Delete.

* Fixed the completely-broken text attribute support for Gecko.

As a reminder: What I care about, and am asking your help for, now is:
Did I break anything as a result of my changes? Preventing regressions
is the most important thing at the moment.

Many thanks guys!
--joanie