[MU3] Export hidden tempo text to MusicXML

[matangover]

Hidden tempo text is now exported as a <sound tempo="XX"/> element without a container <direction> element.

This is now in line with the way MuseScore imports <sound> elements, see: #7171

I changed testSound1 from a test with a reference to a roundtrip import-export test, because it is now fully supported. Some other test reference files had to be changed because they contain hidden tempo markings as well.

• I signed CLA
• I made sure the code in the PR follows the coding rules
• I made sure the code compiles on my machine
• I made sure there are no unnecessary changes in the code
• I made sure the title of the PR reflects the core meaning of the issue you are solving
• I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"
• I made sure the commit message title starts with "fix #424242:" if there is a related issue
• I created the test (mtest, vtest, script test) to verify the changes I made

[Jojo-Schmitz]

rebase needed.

[matangover]

rebase done

[Jojo-Schmitz]

I wonder whether this would need to get ported to master