Merge pull request #1270 from ricksladkey/fixes-1266

Fixes 1266
Merge pull request #1271 from ricksladkey/fixes-1269

Fixes 1269
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