SeratoBeatsImporter: Fix import/export of non-const beatgrids

[Holzhaus]

This should fix the import of Serato BeatGrids where no non-terminal markers are present. The previous code was wrong, I don't know what I was thinking.

[Holzhaus]

I can now export the beatgrid and them import it. However, If I export the imported beatgrid again, the last marker in missing. So there is some other subtle bug in the export code. I'll investigate tomorrow.

[Holzhaus]

Yeeeees, I think I finally fixed it. Sorry, had to rewrite some parts. Please have a look and tell me when I should add comments. At least we now have proper round trip tests that check for correctness.

[Holzhaus]

Please also test this manually. Testing instructions:

1. Check the Serato Metadata Export field in the library preferences
2. Create a beatgrid/beatmap that you can distinguish from the default analyzer result (e.g. shift it or let the analyzer create a non-const beatmap and check "Assume constant tempo afterwards)
3. Export the file tags and restart mixxx
4. Load track to deck
5. Reset BPM/Beatgrid and then reimport file tags -> Is the beatgrid correct?
6. Also: Remove ~/.mixxx and load track to deck -> Does the track have the correct beatgrid even before the analysis finished?

[uklotzde]

Code looks good. My ideas/suggestions for improving safety and readability even further are optional.

[uklotzde]

LGTM