[MU3] Improvement vertical staves adjustment

[njvdberg]

Corrects five issues:

1. Handle the gap above the first staff as fixed gap with a fixed staffUpperBorder spacing.
2. Transfer of extra bracket/brace space to gap between systems. This ensures the gap between staves always use the greatest factor.
3. Prevent adding extra space in phase 2 (make gaps between staves as equal as possible) to first staves only. Now, if the space is left, phase 2 stops and leave it to phase 3 (divide remaining evenly (wih taking the factors into account) over all gaps.
4. When a page was edited, all following pages were laid out again, causing a performance issue on large scores.
5. An optimisation in the second phase of the vertical staves adjustment prevent adding extra space to gaps which don't needed.

Resolves: https://trello.com/c/lAsD7xUf/59-distorted-spacing-between-topmost-pair-of-staves-on-some-pages

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