More work on fixing caret navigation support for Gecko content
* Word navigation should be working as expected
* Line navigation should be improved wrt reliability
* Performance should be improved
* Diagnostic messages regarding broken aspects of Gecko's AtkText support
  added to debugging output

Important: This is still 1000 different kinds of unstable. I've already
branched for 3.14. Choose your branch wisely. :)
Eliminate a couple of cases where caret nav gets stuck in browse mode
New and updated tests
Remove the hack to find things on the same line which Gecko claims are not

This hack causes far more false positives than fixes
Updated regression tests to reflect the latest fix