Support text links with VoiceOver

Now links in text (e.g. links to RFCs after installing the Hyperlink Helper
bundle) are announced as such with VoiceOver and it is possible to search for
them with VO-Cmd-L.

TODO:

* launching links with VO-Space is not supported yet, but Enter (Fn-Return)
 does the same thing and is even more reliable.
* link URL is not supported, so it is not possible to learn the URL
 with VO-Shift-U.

Also I noticed when testing the accessibilityHitPoint with Accessibility Inspector
that the frame reported by `layout` in the `links` construction method (line 1476) is always
wrong in the x-axis - `layout->rect_for_range` returns a `frame` with `frame.size.width == -1`
and `frame.origin.x` some small value.

The y values (`frame.size.height` and `frame.origin.y`) appear to be correct.
What is strange that then the code for AXBoundsForRange (line 1338) also calls `layout->rect_for_range`
again for the same range and reports the correct frame. Any thoughts on what this
might be caused by before I investigate more? Could I be catching the `layout` in some
inconsistent or semi-updated state?

As always, I release the patch to the public domain. Thanks for your review.