Fix hicolor and pyside6 deps

Martin Rysrequested to merge
c0rn3j/syncplay:main into main

The optdepends for various media players hide the icon error and turn it into a warning for hicolor, but it is a hard dep.

PySide2 is not used anymore*, PySide6 is.

* Git master has some seemingly "if macOS" check that ends up using PySide2, but it seems irrelevant to Linux

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