Partially working 'backspace=start' handling.
Fix adjusting for start point.
Fix most tests.  Redo involving deletion not working.
Revamp from starting point to starting offset.
Rewrite context-dependent to context-independent.
All existing tests pass.
Adjust naming of Ctrl+U test to match implementation
Caret with mouse and unit test refactoring

- Fix caret movement via mouse interrupting current edit
- Move backspacing tests to their own class
Clean up up backspacing edits and add unit tests

- Use undo transaction for backspacing edits.
- Add backspace over character unit tests
Document new InsertUtil members
Add backspace over word tests
Restore original TextChange newline handling
Add multi-line tests with repeated backspacing over words and lines
Use tracking line column for start point
Beep don't fail and revert failure check in RunInsertCommand

- All backspacing commands beep but do not fail
- No longer need RunInsertCommand to check for failure so revert
Match vim setting overrides if there is no rc file
Merge pull request #1276 from ricksladkey/fixes-backspace

Full support for 'backspace' option