Show slice selection, ideally before trigger

[odevices]

Requested here:

https://forum.orthogonaldevices.com/t/v0-7-x-progress-and-feedback/6139/27?u=odevices

@tmfset suggested to do the calculation in the UI thread.

[tmfset]

One way to share code here that I've been experimenting with is these interface classes into the DSP object https://github.com/tmfset/er-301-custom-units/blob/c9a846eea9ec8dca800c5158bf63c9e1f887534d/common/graphics/interfaces/all.h

The idea is to expose some calculation or data that can be used to render the object in an abstract way. Allows sharing the same implementation of certain functions to an extent.