FR: Add SWS/NF: Toggle swing grid in MIDI editor

[Breeder]

It would be nice to have a toggle action for swing so it completes the whole MIDI editor action set which currently consists of:
SWS/NF: Toggle triplet grid
SWS/NF: Toggle dotted grid

[juanriccio]

... and groove grid!

[StephanRoemer]

Yes, +1!

[nofishonfriday]

added in v2.12.0