[Meta] Fix Qt scaling issues on X11

Bugzilla report: https://bugs.kde.org/show_bug.cgi?id=356446

To-Dos

• Provide an option or a sugeestion in KScreen to (guide the user to) enable "PLASMA_USE_QT_SCALING"
• plasmashell doesn't properly detect the primary screen setting on login when using PLASMA_USE_QT_SCALING=1

Related MRs

• plasma-workspace!126 (merged)
• plasma-workspace!91 (merged)
• frameworks/kwindowsystem!2 (merged)
• frameworks/plasma-framework!301 (merged)