Rudimentary undo line implementation
Handle undo line after movement and swap saved line
Clean up line change tracker and add undo line unit tests
Fix typos and handle uninitialized text view
Beep don't fail and handle TextViewLines exceptions

- If no line undo information is available just beep, don't fail
- Wrap access to TextViewLines in try/with block
Merge pull request #1279 from ricksladkey/fixes-line-undo

Add support for the 'U' command (undo latest changes on one line)