Fix indentation of KCM pages, add text to advanced page.

Explain in more detail what the "show hidden files" option does, there's room for it so why not be friendly to people who might not know what this is about.
Add option to generate par2 recovery info for bup backups.

Possible improvements would be to check if par2 is indeed available, and to detect failures in file digger and automatically try to repair pack files if par2 is available.
Closes #19.