to wrap io.popen and os.execute system calls
respectively. These wrapper functions provide
the necessary quoting on windows to get handle
strings with spaces and special characters.
- rename OS_PATH_SEPARATOR -> PS and use in all needed places
- change function name from `open_file` to `launch_default_app`
- move `mkdir`, `rm` and `launch_default_app` helper functions to libs
- add executable_path_widget to export module if needed, not to the
preferences
- add show_status
- remove support_format
- change function name `format` to `format_string`, add comments
- replace apostrophes by quote marks