(fix) avoid catching Num key modifier on macOS

[ronso0]

fixes #13305

59b54da introduced a regression on macOS:
debugging in #13331 revealed that on macOS the arrow keys are sent with Num modifier, for which there would be no match in our keyseq/control hash.

Revert to checking the individual modifiers and cosntruct the QKeySequence with QStrings.

[ronso0]

Note: alternatively we could also check for
if (macOS && Num mod && (key == Up || key == Down || key == Left || key == Right))
and remove the Num mod in these cases.

Not sure if that's worth the effort.

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And I'm not sure whether we should explicitly check for the Num mod in general. This would allow to use the numpad for shortcuts, too. But that's separate from this rather urgent bugfix.
Please someone check this and merge asap. Thanks!

[m0dB]

I confirm the bug and the fix (I had no idea that shift left / shift right was supposed to load the track, very convenient indeed!).

[DJMaxergy]

Tested and works for me on MacOS, thank you :)