Update getChildAtOffset() to use hypertext.getLink()
Update getChildAtOffset() to use hypertext.getLink()

This is the correct way to do things and seems to (now) work for Gecko.
In addition, using hypertext.getLink() is needed for Chromium because
it exposes more children than are hyperlink objects. If this introduces
a regression, we can restore the original behavior, but just for Gecko.

GNOME/orca,GNOME/orca,GNOME/orca,GNOME/orca