[MU3] Fix #318326 - System Text Line missing in existing parts when added via drag&drop to non-top staff

[njvdberg]

Resolves: https://musescore.org/en/node/318326

If a TEXTLINE is dropped in a ScoreView, set firstStaffOnly to true if the dropped textline is a System TextLine.

PR #7761 is a similar PR for master.

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