Use platform detection in SystemSettings

Summary:
SystemSettings needs to run on Wayland in a Wayland session and on X11
in an X11 session.

Test Plan:
unset QT_QPA_PLATFORM and XDG_SESSION_TYPE -> x11 window,
reset XDG_SESSION_TYPE to wayland -> wayland window

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11445