Add support for pregap tracks

[ruippeixotog]

This pull request adds support for pregap tracks in imports from Musicbrainz. This will only work after Musicbrainzngs releases a new version and beets starts using it, but everything should work as normal until then.

Two things to note:

• I opted to leave track_info.index starting at 1 even in the presence of pregap tracks, which means that users with per_disc_numbering deactivated will see the pregap track as track 1 and every other track as its "original" album track number plus one. In albums with a single disc that felt strange to me, but then I thought about albums with multiple discs and multiple pregap tracks - I could set the index of the pregap in the first disc as 0, but then I would have to go sequential from there, including other pregap tracks in the same release. As I thought that would be rather inconsistent I implemented as seen in this pull request, but suggestions would be welcome;
• I'm not that fluent in Python, so please tell me if there is a more efficient/pythonic way to do this :)

Fixes #1104.

[sampsyo]

Since this is a little tricky, an inline condition, like this:

item.track = track_info.medium_index if track_info.medium_index is not None else track_info.index

or a comment to the effect of "we want to let the track number be zero, but if the medium index is not provided we need to fall back to the overall index" would be helpful here.

[ruippeixotog]

I did not inline the conditional because of its size; I'll add that comment there instead.

[sampsyo]

This looks great, and I agree with your reasoning about the per_disc_numbering distinction. It makes sense to make those all just indexed serially from 1. Would you mind adding to the documentation some discussion of that behavior?

Other than that (and pending a changelog entry), this looks great! Let me know when you're happy and we'll merge it.

[ruippeixotog]

No problem, I'll describe the numbering behavior in the documentation and add a changelog entry if you want. Where in the documentation do you suggest I add that?

[sampsyo]

Good question. Certainly on the config page under per_disc_numbering, but perhaps we'll come up with somewhere else too.

[ruippeixotog]

Sorry, I messed up with the local branches here. I added some text to the per_disc_numbering, please check if it looks good to you, as it's mostly a question of text style.

[sampsyo]

Awesome; looks great! Thank you for addressing this feature, even though it spans two projects. ✨

I expanded the docs a little; let me know if I messed anything up.

[pdf]

I'm not sure I agree with the decision to number always renumber all tracks if there's a pre-gap track (if I understand correctly what you've implemented here). Ripping tools will generally only produce a single file for the pregap, but I'm not entirely certain how to handle metadata that includes multiple pre-gap entities.

Ignore my rambling, misunderstood.