Merge certain effects across voices (e.g. let ring & palm mute)

[Danielku15]

Currently alphaTab renders the effects like let ring and palm mute individually per each voice. While this is not correct, it rather bloats the music sheet. Guitar Pro 7 and 8 also merge those effects across voices to be shown only once.

Additionally I noticed with this example that let ring doesn't expand on tied notes which seem to be the case in GP7 and GP8. Might be a bug. We should extend line effects like this to tied notes.

[Danielku15]

Maybe in the same step we can also adapt how "effect slots" are reused. Guitar Pro seems to keep it simple and only extend a slot reservation if the effect is really spanning across bars without interruption. This would allow a lot more condensed notation in case of horizontal layouts

alphaTab:

Guitar Pro: