So far we didn't try to do high DPI on kwin internal windows, such as
the user context menu and tab bars and whatever.
Due to wayland scaling they were the correct phyiscal size but upscaled.
This patch fixes our QPA to enable Qt's high-dpi support.
Note icons are still low res. This is because the global
QGuiApplication::devicePixelRatio which is the max of all connected
screens is static for the duration of the app. QIcon uses this when
determining the DPR to use. This will require Qt changes.