Add Qt6 support for Kvantum

[Lucas Varsky]

When you have qt6ct and kvantum installed, what would usually happen is that qt6ct shows kvantum as an available option in the drop-down menu in the "appearance" tab to use to theme QT programs (the correct behavior that qt5ct shows).

Instead, qt6ct simply ignores that kvantum is an option.

Reproducible: Always

Steps to Reproduce:

    install the "qt6ct" and kvantum "packages"
    launch qt6ct
    try to find the kvantum option in the drop down menu inside qt6ct's appearance tab.
    Actual Results:
    No kvantum option appeared in the dropdown menu.

Expected Results:
You should see the kvantum option in the dropdown menu.

I suspect that this bug is related specifically to qt6ct because kvantum can theme programs without problems if it's selected as one of the theming options inside KDE's appearance settings.

This problem is also present on Fedora, but not on other distributions that i tested like Arch or NixOS, which leads me to believe it's a packaging problem.

Related Fedora bug report i opened:
https://bugzilla.redhat.com/show_bug.cgi?id=2263614

[Lucas Varsky (raxelgrande)]

Turns out this isn't a qt6ct issue, but a Kvantum one.

After talking with Kvantum's main developer, Tsu Jan, we found out the problem.
To give Kvantum the ability to theme Qt6, it needs to be packaged alongside its Qt6 support plugin.
Ubuntu already does it well for Qt5, specifically packaging kvantum for it "qt5-style-kvantum".

Please, package kvantum with Qt6 support in the same style used (if possible), like "qt6-style-kvantum". Or maybe repackage all of kvantum into a single package that has both Qt5 and Qt6 support.

Related discussion: https://github.com/tsujan/Kvantum/discussions/941

[Peter B (ueter)]

See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1057763