[MU4] Show part name in instrument form instead of instrument name.

[njvdberg]

If opening ChooseInstrumentsPage, display the part name if available and use the instrument names otherwise.

The behavior is now similar as it was in 3.x.

The PR is required for the upcoming score ordering PR which needs part information for the ordering.

• 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]

Is this the for for https://musescore.org/en/node/317264 (and for master) and so the counterpart to #7544? Wasn't #7571 enough?

[njvdberg]

@Jojo-Schmitz "kind of" and "no" 😉
Compared with 3.x the new Instruments form is 100% rewrite and also the underlying C++ code is completely new. But by chance it showed the same problem, instrument names instead of part names. In this sense it is a kind of counterpart of #7544.
And no, #7571 wasn't enough. As a matter of fact it is doing something completely different. #7571 is changing the Instruments Panel while this PR is changing the Instruments form.

[njvdberg]

This PR is included in PR #8099 .