Implement sample slicing and note allocation

[pgregory]

Description

Implement the ability to manually, and possibly automatically, slice a sample into sub-samples with an instrument in each. For example, to split a drum sequence into individual drum samples non-destructively, each sub-sample can be played individually via a note on the instrument, but only the single original sample data exists in memory. See Renoise for an example...

https://www.youtube.com/watch?v=yhhUY7FKJds&start=35

[pgregory]

Suggested by @H3ADL3SS on the Tracker Music Discord.