(fix) adjust menubar hotkey detection (Alt) for Qt 6.9

[ronso0]

Fixes #15350

With Qt < 6.9 Alt press was reporte as Alt(modifier) + Qt::Key_Alt, now it's just the latter.

@Swiftb0y

[Swiftb0y]

Works, thank you. No regressions on your side too?

[ronso0]

Yup, still working.
getKeySeq has already been looking for key = Alt, so no issues to be expected 🥸

[ronso0]

maybe @JoergAtGithub @Eve00000 can test on Windows just to be sure?

[JoergAtGithub]

I can only test with Qt6.8

[ronso0]

That would help to verify nothing is broken on Qt < 6.9

Though, we can also wrap it in #ifdef Qt >=6.9

[JoergAtGithub]

I don't see a difference between 2.5 and this PR - both with Qt 6.5.3.
But I noticed that the menu toggle only works reliable after restarting Mixxx once. Immediately after setting the checkbox in the View menu, ALT just hides the menu, but can not show it. This happens in both cases.

[ronso0]

Okay. (not)
Can you please file an issue so we don't forget about it?

[JoergAtGithub]

Can you please file an issue so we don't forget about it?

#15385