Skins: Fix checkbox styling on Qt 6

[fwcd]

Fixes #11630, fixes #11908

This fixes the UI regression that the BPM lock and play count checkboxes are rendered twice when building with Qt 6:

Before	After

Seems to solve the issue under all skins and I couldn't notice any other UI regressions. Perhaps someone could test the Qt 5 build too, just to make sure that this doesn't break anything there?

cc @ronso0

[JoergAtGithub]

Looks correct on Windows now. Note, that there is also an additional light grey border/ring on the gain knobs:

For reference a Mixxx 2.4 Qt5 build:

[ronso0]

Does the ring occur with all Qt6 builds (updated QtSvg version) or only this branch?

Re this fix: Qt stylesheets, especially partially overriding default styles is still a mystery after all...
[jack in the box emoji]

[JoergAtGithub]

Does the ring occur with all Qt6 builds (updated QtSvg version) or only this branch?

It occurs with all Qt6 builds on my Windows11 system!

[ronso0]

Alright, I'll take a look and try to tweak some LateNight controls when I'm back.
With Qt5 some Svg effects (translucent gradients) where not rendered at all or produced weird glitches.

[fwcd]

Any update on this? Is the ring issue related to this?

[ywwg]

This fixes it for me!

[ywwg]

It occurs with all Qt6 builds on my Windows11 system!

Since it occurs on all qt6 builds (not just this branch) then that's an unrelated issue we should fix separately -- can you file a new issue for that?

[ywwg]

(in general I am not involved enough with day-to-day mixxx to merge pull requests, so maybe @daschuer you can do that if it looks good to you?)

[ronso0]

On Linux with Qt 6.2.3 this fixes that unchecked unselected checkbox icons aren't rendered at all.
Don't notice regressions with Qt 5.12.8
LGTM, thanks!