Fix #328003 score property crash

[jonnymoo]

Resolves: https://musescore.org/en/node/328003

Following accessibility changes saving score properties causes MuseScore to crash

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[jonnymoo]

The reason why the build failed was I incorrectly put qualifiers on my class declaration - see https://stackoverflow.com/questions/5642367/extra-qualification-error-in-c

I've fixed with the latest commit.

Apparently visual studio is more than happy with this, but other compilers are bit more strict. I've removed them. I haven't put the properties item class back into the properties page and simplified because I struggled to mock out the whole of the page - so it is easier to test the smaller class. If you think this is not good practise, I of course won't take offense but might need a bit of help in how to create a moc screen and inject a project with properties etc.