Add crash recovery, auto save timer option.
Add feature to save undo history to temp file. Once app closes this file is deleted. If app/system crashes and file stays on disk, on next start app will notice the file and prompt user to recover. Whole state and undo history would be restored.
Add an option to perform auto saving every N minutes. It can be setup to:
- save to .bak extension
- overwrite original file
Original issue: https://github.com/maxrd2/SubtitleComposer/issues/113