Sample Track Recording with Jack backend

[mmeeaallyynn]

This adds support for the Jack backend for recording Sample Tracks.

Based on #5990

[JohannesLorenz]

@mmeeaallyynn While the PR diff itself looks correct, the commit list (30 commits by different people) looks bad. Do you mind doing a git rebase master, followed by a git push -f? Another way would be a git reset master, followed by a git commit and a git push -f.

[mmeeaallyynn]

Thanks for the hint, I hope it looks good now.