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.