CMake: Fix Qt resource files with CMake 3.20

[Holzhaus]

Fixes https://bugs.launchpad.net/mixxx/+bug/1921549.

[uklotzde]

Could you provide a short explanation what was wrong and how you fixed it?

[uklotzde]

Ok, the "how" is obvious, by inlining the code.

[Holzhaus]

I suspected somethings was wrong with the QRC file because according to the mixxx logs the resource dir is correct but the Qt Resource System still didn't find any resources.
Since I updated CMake 2 days ago to 3.20, I suspected that this was the root cause.

Before, we added a mixxx-qrc OBJECT target, then added the resulting objects to mixxx and mixxx-test. This is weird and apparently doesn't work with CMake 3.20. Instead, it now just adds the QRC file to the mixxx/mixxx-test target directly using target_sources. More straightforward and it works.

(We can't just add it to mixxx-lib because that would require using the Q_INIT_RESOURCES macro).

[uklotzde]

The new solution is simpler and easier to understand. Removing the remaining redundancy would require to fix mixxx-lib as the comment suggests. LGTM

[uklotzde]

Thank you for the quick fix!

[uklotzde]

Fortunately even Fedora f35/rawhide still uses 3.19.

[Holzhaus]

Thank you for the quick fix!

I had to, otherwise I can't build Mixxx anymore 😜

[jospezial]

This change also fixed an issue with missing button icons that I had in Gentoo since 2 months with cmake-3.19.x.
Mixxx in fedora rawhide from rpmfusion package also has the missing button icons.