Rewrite many of the utility methods for accessing Gecko content

Fixes:
* Performance through caching return values of frequently-called methods
* Present "link" when arrowing into text descendants of links
* Display inferred labels for radiobuttons and checkboxes in same
  order as if they were proper labels
* Increase consistency between presentation of ARIA and native apps
* Eliminate more unnecessary braille refreshing
* Stop presenting content from the wrong document frame
* Stop presenting images which are not rendered and have no alt text
* Stop double-presenting inferred labels in SayAll
* Present "link" as part of the same utterance when navigating by line
* Fix bogus duplicate characters in braille
* Fix treating inline block spans displayed on a single line as if they
  were on multiple lines

Also remove UI tests for dialogs which no longer exist in Firefox.