shell-app: Improve discrete GPU detection

shell-app: Improve discrete GPU detection

The original logic only took into account if a GPU is the "Default",
meaning the GPU used during boot, which was intended for Laptops with
Hybrid Graphics.

The new logic uses the new Discrete key to figure out which GPU is the
most appropriate to use in order of:
- the first non-default GPU if it is discrete
- the first discrete GPU if it exists
- the first non-default GPU

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3193>