partially revert #3902 Skins: fix qss icons with kIconThemes 5.80

[ronso0]

because with scale factors >100% the (vector) icons set with 'background[-mage]' are not truely scaled up, only their 100% pixmap, which results in blurry/pixelated icons.
Users still with broken libKIconThemes5 v5.80 have to live with squeezed icons until they (can) update that lib.

https://mixxx.zulipchat.com/#narrow/stream/109171-development/topic/2.2E3.20planning.3A.20https.3A.2F.2Fgithub.2Ecom.2Fmixxxdj.2Fmixxx.2Fprojects.2F2/near/240843338

[ronso0]

voila, more time wasted. looking forward to fix the conflicts with main

[ywwg]

can confirm this results in squeezed images on ubuntu 21.04

[Holzhaus]

I can confirm this fixes the pixelated icons with double scaling. It also fixes the square BPM Slider center indicator in LateNight (this even happens without scaling). Merge?

[ronso0]

I couldn't simply revert the PR because it contained other fixes, and because I pushed another small fix later.
I'm not worried it works, im just a little worried that I also restored all button colors.
I seriously don't want to touch qss again before the release, so I'd like to be sure it's complete. testing a few buttons in every skin region should suffice.

[ywwg]

with the new deb from the PPA, skins look good on ubuntu. ok to merge by me

[ronso0]

I found some issues like double preview icons in the library (from default.qss and skin style)
am about to fix those. since no one else noticed anything weird I'll simply merge when I'm done.

[ronso0]

double preview icons in the library

fixed of course after rebuilding.
though there was a wrong icon in the Deere sampler.

D O N E !!!

[ronso0]

stupid me,. I had two 'revert' branches and applied that small fixes to the wrong one. I'll push those directly.

[ronso0]

and I'm merging to main now