Follow-up from "Add support for XWayland client scaling"

The following discussion from !56 (merged) should be addressed:

@ilyafedin started a discussion: (+9 comments)

Note: Xft/DPI is also read by Qt, so this may affect text scaling in Qt applications as plasma-workspace also reads the following setting: https://invent.kde.org/plasma/plasma-workspace/-/blob/master/kcms/fonts/fontinit.cpp#L30 while kde-gtk-config is not, Qt prefers Xft/DPI over Xft.dpi just like gtk. Although I'm not entirely sure how really it affects Qt as plasma-workspace also sets QT_SCREEN_SCALE_FACTORS environment variable meant for debugging.

What we have to do:

• Read the base text DPI from a config file (either kcmfonts or kdeglobals) instead of hard-coding 96 DPI
• Set Xft/DPI in XSettings to match the Xft.DPI resource (multiplied by 1024, of course)

Also: is it possible to have proper XWayland client scaling for Qt apps using the xcb QPA platform?

BUG: 466463