Qt 6 support

[ArtyIF]

Qt 6 has been out for a while, and some popular apps like qBitTorrent have started upgrading to it. Unfortunately, neither Breeze not Lightly support it right now, so they fall back to Fusion theme and end up looking like this:

The situation is a bit better with a light color scheme, but the look still doesn't match the rest of the system. It would be nice if the theme was upgraded to support Qt 6, either before or after Breeze adds the support.