B&B editor overhaul

Nobody has claimed this yet.

### Enhancement Summary

Currently any advanced functionality in b&b editor requires editing in piano roll which I don't really have any issue with despite the fact it looks ugly and requires additional navigation.

### Summary:
Make B&B editor more akin to piano roll with a drum kit instrument, but where you can set instrument per tone yourself.

### Target use cases:
- Off-beat notes (incl. 1/32 notes, 1/12 notes, etc.)
- Beats with set duration (e.g. sweeps, etc.)

### Implementation Details / Mockup


This is a crude mockup of how I see this. non-tone-shifting tools from piano roll should also be included. This particular design, however, ignores the existence of infinite notes --- the thing b&b is originally for. I'm not that deep into UI/UX design, but it could be denoted with either being just a rod [maybe with a trailing fading out gradient to distinguish from a short note] and/or having different color.

### Please search the issue tracker for existing feature requests before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

Start by finding the B&B editor and piano roll entry points in the LMMS desktop application, then compare their existing behavior with the mockup and listed use cases. Done means defining and implementing an editor design that supports off-beat notes, set durations, per-tone instruments, and infinite notes.

Written by the indexing model from the issue text.