Use nio Files/Paths

[t-ober]

    I think it could make sense to work with `File`s here, as you can extend them like this: `File file2 = new File(file1, "subdir");`. This would also add type safety (files are recognizable by type, as they're not just represented by Strings). I'd suggest opening a new issue for this

Originally posted by @sebastian-peter in #79 (comment)

[sebastian-peter]

I adapted PathsAndSources to use nio Files/Paths in #75. These are preferable over using legacy java.io.File. There are some spots in this project left where Strings are still used and could also be replaced by paths.

[t-ober]

Resolved by #75

[sebastian-peter]

As I said in the previous comment, there's many places where we still use Strings and/or java.io.File, so this is not solved imho