Values in sp unit don't remain unchanged after changing spatium
Windows 10, GIT commit: de38cce
Discovered while working on #187151: Hairpins shifting out of position on systems containing ties from stem. Not a recent regression in general - this goes back to before release of 2.0. For hairpins in particular the problem was introduced a little later than the others, but it was definitely there by 2.0.3.
1) create a score with a time signature, a two-note quarter note chord with an arpeggio, and a hairpin
2) apply a user offset to the time signature, chord stem, arpeggio, and hairpin
3) change spatium value
Result: the user offset for those elements is altered. Basically, we aren't scaling them, because we override spatiumChanged() but don't call Element::spatiumChanged() (or whatever the parent is).