Fix balloons on cross-staff grobs

Since balloons have a height that depends on their parent, they must marked cross-staff when their parent is cross-staff (at least when the Balloon_engraver is not in Score context, as if it is, the grobs won't participate in their parent's axis group in the current implementation).

For now, BalloonText is the only sticky grob with a cross-staff callback. Footnote, ControlPoint and ControlPolygon do not need it because they have extents set to #f (but testing coverage is added nevertheless). The case of Parentheses is more tricky because handling it correctly would requires something like a version of ly:relative-group-extent that does take cross-staff grobs into account (issue #4892); for now, it is left as-is.

Closes #6161 (closed).