Please sort XML tags reproducibly (Mk Ⅱ)

From #274166, with new data inline:

Please sort the order in which the XML tags are emitted in a reproducible manner.

Case in point, I was just loading all my scores (last saved with 2.2.1) in 2.3.1 and saving them, then reviewing the .mscx diff, and I got quite some delta for some scores.

Inside a ChordList tag’s font subtag, the mag subtag is always first, but the following sym subtags seem to come in arbitrary, random order instead of sorted.

NEW: Example attached: when I save Requiem-orig.mscx I get Requiem-save1.mscx. Then I exit MuseScore, load Requiem-save1.mscx, save, get Requiem-save2.mscx. These files all have only a small diff, but that’s annoying.

(I’ve revisited the font stuff in a different issue.)