Add CueInfo DTO

[Holzhaus]

This adds a CueInfo class that does not have any dependencies on Track or Cue. To archieve this, we use positions in milliseconds instead of sample positions, so that we don't need to know the tracks' sample rate.

The Track object gets a new method to import cues from a list of CueInfo objects.

This would be useful for Serato Markers Integration (PR #2499).

Also see related Zulip discussion for details:
https://mixxx.zulipchat.com/#narrow/stream/109171-development/topic/2.2E3.20planning/near/189265725

[Holzhaus]

@uklotzde Sorry, I fixed up the commits. The only thing missing is a decision about moving the constants into global namespace: https://github.com/mixxxdj/mixxx/pull/2523/files/d0fb17ca6faaf4c59d58067f826d32de55eef187#diff-1483dfb71fc243bb08cf3cb33a1d5c5c

[uklotzde]

@Holzhaus Rebuild this PR as needed and deliberately drop any intermediate versions that are not worth keeping in the final commit history.

[Holzhaus]

@Holzhaus Rebuild this PR as needed and deliberately drop any intermediate versions that are not worth keeping in the final commit history.

Done ;-)

[Holzhaus]

By the way, should I put this class into the mixxx namespace? Some classes are and some aren't and I don't know what the pattern is.

[uklotzde]

By the way, should I put this class into the mixxx namespace? Some classes are and some aren't and I don't know what the pattern is.

I've just started that once. It helps to prevent name clashes with external libraries. It is still not consistently applied, yet. Yes, maybe that would be an option for such a foundational component.

[Holzhaus]

By the way, should I put this class into the mixxx namespace? Some classes are and some aren't and I don't know what the pattern is.

I've just started that once. It helps to prevent name clashes with external libraries. It is still not consistently applied, yet. Yes, maybe that would be an option for such a foundational component.

Done.

[Holzhaus]

If nothing else is missing, let's merge. I can rebase #2499 on master then.

[Holzhaus]

Done.

[uklotzde]

Ok, I guess this is ready now.

[uklotzde]

LGTM. Thank you, Jan, for getting this rework started!