Fix 302571: Disappearing beam while editing slur node

[njvdberg]

Resolves: https://musescore.org/en/node/302571

The cross measure beaming algorithm takes system breaks into account, but only when the
first chord of the cross measure beam was part of layout range.
When the layout range starts with measure contains mid/end notes of the cross measure
beam, the break in the cross measure beam due to a system break was recognized when this
was an explicit system break only (e.g. by placing System or Page Break element). Any
system breaks generated by the auto-placer were ignored. As a result a partial layout
(e.g. triggered by an edit) would remove the hook of the "broken" cross measure beam because
the auto-placer assumed it was beamed to a note in the previous measure.

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