Fix #316025 - Fixed spacers do not function with vertical justification enabled

[njvdberg]

Resolves: https://musescore.org/en/node/316025

When a system contains a fixed spacer, the gap above that system will become a gap with a fixed gap and no extra space will be added to this gap.
To implement this, System::minDistance() will now, instead of just saving the fixed distance, save the fixed spacer instead. distributeStaves() will use this to find out whether a system has a fixed spacer or not.

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[MarcSabatella]

Thanks for this! I had taken a quick look hoping there was a simple two-line fix somewhere, but soon realized it would likely mean a change to the VerticalGapData, and at that point realized I was in a little my head as far as feeling confident I'd get it right. So I can just say, this looks like the right sort of fix. Will do a test of the result and report back.

[MarcSabatella]

Seems to work beautifully!