[orca-list] Text selection rewrite has begun (and kinda sorta works in Gecko)

[Joanmarie Diggs <jdiggs igalia com>]

Hey all.

I have just committed some more unstableness to master: The
not-yet-finished rewrite of Orca's support for text selection. If you
notice anything severely broken that wasn't broken before, please let me
know.

On a related note, as the subject indicates, text selection now kinda
sorta works in Gecko. Here's the deal:

* Because of Gecko's broken native caret navigation, Orca has to
  override it with its own, but:
* Because of Gecko's broken accessible text-selection setters,
  we cannot similarly override that with any degree of reliability.

As a result of the above combination of issues, if you select text using
Orca, Orca stops controlling the caret and does its best to present what
Gecko is telling it about the text selection. If you are just in
ordinary text this largely works. It currently fails in two places:

* Crossing from the middle of one object into the middle of the other.
  For instance, if you're inside a heading and shift down arrow and
  land in the middle of a link in the middle of a new paragraph, all
  bets are off.
* If, after selecting, you resume arrowing, Orca's keyboard navigation
  kicks in and moves the caret within the content, but Gecko doesn't
  clear the selection in response.

I'm still working on fixing those (or reporting them if they are yet
another Gecko bug).

Again, unstable master is unstable. But I think the changes I've made so
far are functional enough (or not hopelessly broken) for those
interested to give it a spin while I continue working on this.

Thanks and take care.
--joanie