fix #313420 [MusicXML] Instrument brackets are all imported at column 0

[AntonioBL]

Resolves: https://musescore.org/en/node/313420

System bracket implementation was changed between MuseScore 2.x and 3.x, but it was never updated in the MusicXML import.

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[AntonioBL]

There was a problem with Create_New_Score.mscz during rebase.
I will try to amend it.

[AntonioBL]

Now the rebase conflict is solved.
That mscz file messed a little with the git history of my branch.

[Jojo-Schmitz]

Blame it on @njvdberg ;-)

[njvdberg]

Blame it on @njvdberg ;-)

I just opened a new packet with brown bags, shall I one or two for myself? 😉