wrap a windows command in quotes. Added a sanitize step
to windows_external_command() plus quote_windows_command()
to deal with all the windows username possibilities
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
