[Meta] Fix Qt scaling issues on X11
Bugzilla report: https://bugs.kde.org/show_bug.cgi?id=356446
## To-Dos
- [x] [Provide an option or a sugeestion in KScreen to (guide the user to) enable "PLASMA_USE_QT_SCALING"](https://mail.kde.org/pipermail/plasma-devel/2022-February/121690.html)
- [x] [plasmashell doesn't properly detect the primary screen setting on login when using PLASMA_USE_QT_SCALING=1](https://bugs.kde.org/show_bug.cgi?id=450443)
## Related MRs
- https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/126
- https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/91
- https://invent.kde.org/frameworks/kwindowsystem/-/merge_requests/2
- https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/301
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