Improve fretboard note playing indicators

[1j01]

Currently it looks broken, like it's got an offset that's not supposed to be there.
It is actually intentional, it's offset so it overlaps the fret that that would be contacted physically.

It should be something more like a circle (where you'd put your finger), blobbily connected to an oval on the fret, or a highlight of the fret. It might look like a bombshell, which might be weird. Maybe it shouldn't be connected? idk
The highlight that represents a finger should move along with your mouse / finger / etc. (and be centered for playback, or if articulations are implemented, it could slide and stuff too)

[1j01]

Improved a bit in aad53a3

It definitely looks weird and complicated and like a bomb if I do this: