support sequence playhead reset

[dkrieger]

Feature

support sequence playhead reset for:

1. clip
2. row
3. (stretch goal) song / all active clips
4. (stretch goal) same as above, but inclusive of audio clips

Background

It would be nice to be able to trigger playhead reset, similar to triggering reset on a sequential switch in a modular environment. This would be another way to squeeze more variation out of a given sequence, much like how probability, ping pong, and variable row/clip length polyrhythms currently provide.

An alternative approach would be rotating a clip/row to move the start to the current playhead position, but this would be destructive and present performance issues.

Implementation

UI

1. Hold mute/launch and press play to move the playhead back to the start (note: relates to Improve UI for selecting rows in kits (and subsequently synths/midi/cv) #2126 )
2. (stretch goal) use other clips' reset/loop actions to trigger reset (e.g. when a particular drum clip starts over, reset the longer bass clip)

Notes

Unclear how this should quantize, but I'd propose rounding to the nearest step, i.e. if you're closer to the next step, wait until that step, else move the playhead back to to step 1 + the offset of the currently playing step, where the step is defined by the current zoom level (note: not familiar with implementation details, this might be an incorrect way of modeling the problem, but hopefully the intention is clear).

I'm willing to work on this feature, but may need some guidance for the UI code (and in general as this will be my first contribution), and it may take some time for my schedule to open up to work on this.