shell-app: improve discrete GPU detection
Adds proper discrete GPU detection through switcheroo-control with a fallback to the old behavior (using the first non-default GPU).
required switcheroo-control merge request: https://gitlab.freedesktop.org/hadess/switcheroo-control/-/merge_requests/69
a Fedora COPR is available with the switcheroo patch for testing.
KDE counterpart: https://invent.kde.org/frameworks/kio/-/merge_requests/1556