Replace `os.path` with `pathlib`

Paths are currently handled mostly using the os.path module. However, we can only benefit from a move to the pathlib library. It's rationale is explained well in 1 and 2.

Note, that in doing so we should also add the ability of storing paths in the database as relative to the user's home! This will allow easy transitions to/from other machines using different user names. This will also ease/enable sharing a coBib database in a multi-user environment.