added:
- ROUNDtCLOCK: Game Modes added: choose how many times a segment have to be hit; DOUBLE and TRIPLE are possible hits now
- statistic: let user select filename and location for export
- raised min. Android Version to 5.0 ("Lollipop" - API 21)

fixed:
- huge code refactoring; translation of non english elements and some modularization
- main menu: number of players were lost, if selected game typ was switched away from SET/LEG
- statistic: export: fixed "no permission" error on Android 10 etc.
- statistic: import wasn't always possible due to message "Error: This is not a Dart Checker export file."