Fixes and refactorings for Editor on Firefox.

Fixes Editor_a11y.html regression test failures and removes firefox specific code branches:

- make FF use contentEditable (like other browsers) rather than designMode
- make FF use nested <div> inside of <body> (inside of <iframe>), just like other browsers
- expand tabbing helper code (both for tabbing/shift-tabbing into and out of the iframe) to run on all browsers, not just IE
- reduce the number of branches in focus(), so that chrome and FF use same path

Fixes #10415, #17560.
avoid spurious failure on chrome where italic button is checked,
and stop using internal editor method to position caret, refs #9460
Remove trailing whitespace from Editor value.
Fixes test failures on Chrome in Editor_mouse.html, Editor_a11y.html.
Fixes #17561.
mozilla changes for contentEditable
Add test case for clearing editor then focusing it.  Refs #10268.
Remove _cursorToStart code that's apparently no longer needed.  Fixes #17568.
Remove no longer needed FF code, refs #11898.
remove mozilla &nbsp code