Allow control of rest positioning

[Danielku15]

See #2559

Current Behavior

As of today alphaTab purely controls the rest positioning itself using standard rules and collision detection.

Needed Behavior

Users should be given custom control on which "pitch height" rests should be shown. This should be an opt-in feature of course.

MusicXML has some feature to control the display step and octave:
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/rest/

For Guitar Pro we'd have to check if it might contain the same information.

For alphaTex we need something custom (beat effect).

Related problem

As seen in the discussion: if the rests are on the main voice, but notes are placed on the secondary voices, the rests are not displaced automatically on the collision detection. We might need to change the collision detection logic to displace rests after all voices have been layouted.

If a custom position is set this should overrule collision detection.