Loop: quit slip and adopt rolling loop

[ronso0]

For now this is an experiment for personal use, but I want share it.
Based on #12475, so this is just the last commit.

This fixes #9445 for me when using my Reloop TerminalMix2 or ~4

• press and hold a looproll button for a transition, or just as filler
• notice I want to play with that loop, or simply need my looproll finger for other stuff, instead of holding the looproll button for the entire transition
• press beatloop_activate
  => loop becomes a regular loop, slip mode is deactivated (no seek!)
• release looproll button (nothin happens)
• do whatever I like with the loop

[github-actions]

This PR is marked as stale because it has been open 90 days with no activity.