Use pathlib everywhere in beets

Pathlib is awesome. It abstracts path the platform for path manipulation. It would allow us to have more robust code and get rid to all the calls to `beets.util.*_path()` functions. It could be extended to provide desired `repr()`.

I feel like working on it. It is _huge_ though.
- Am I missing a delicate point that prevents us from using it
- Should it be a 1.3.12 milestone?

The issue proposes replacing beets.util.*_path() calls with pathlib across beets; start by reviewing those path helpers and the places that use them. Done would mean the path manipulation has been migrated broadly enough to remove the existing helper calls while preserving platform-robust behavior.

Written by the indexing model from the issue text.