shell-app: improve discrete GPU detection

Jan Drögehoffrequested to merge
Jan200101/gnome-shell:switcheroo-discrete-detection into main

Adds proper discrete GPU detection through switcheroo-control with a fallback to the old behavior (using the first non-default GPU).

needs switcheroo-control 3.0 (or later)

KDE counterpart: https://invent.kde.org/frameworks/kio/-/merge_requests/1556

Edited by Jan Drögehoff

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