Assorted improvements

[dhewg]

This PR…

...is just a bunch of smaller things I noticed while starting with this nice tool. The commits are small and easy to review (I hope).

Some of it is mostly internal stuff, the nice user-facing change is that -tag_wahoo_xml now accepts random filenames, and not just relative ones to the internal resources directory. Similar, there's now --tags_to_keep and --tag_transform with the exact same logic.

As most major themes will touch all three of the mentioned files, it's now possible to have multiple themes, each with their own configuration files, next to each other and build them without messing around in the pip install or user config directories.

Caveats

I didn't touch the unittests nor the gui. The former will have to be adapted if these changes are accepted. The latter should still work, but could be extended to handle the new command line switches.

Pull Request Checklist

• [ ] Reviewed the Contributing Guidelines
  • Especially the Pull-Requests section
• [x] Commits (and commit-messages) are understandable
• [x] Tested with macOS / Linux
• [ ] Tested with Windows

[dhewg]

The old pylint version didn't work on my end, so I updated that. With that, every commit now passes the pylint checks. Most unittest pass now, some do not (like that @mock thing, but that didn't work before either here), but I'd leave it at that...