Allow lyrics to be vertically centered between staves

[ajuncosa]

Resolves: #33749

• Allow lyrics to be vertically centered between staves.
• The staves must either belong to the same part (e.g. grand staff) or both be vocal parts.
• Dynamics can now also be centered between vocal parts (before they were only centered in a grand staff of the same part).
• All lyric verses for a system, as well as the centerable items that stack vertically with them, will be centered as a single rigid block to keep the alignment and distances computed during previous layout steps.
• It is still not possible to stack non-centered items outside of a centered item (the item will not be centered at all).
• Stacking order has changed: lyrics are now before dynamics (so ... => lyrics => dynamics => measure numbers => octavas => pedals => ..., whereas before they would have been after pedals).