Make gtk3 patches work properly

Currently, the sandboxed version of gtk3 can't read the property is-phone from the host system. I work around for this is to manually set the property after installing the runtime.

flatpak run --socket=wayland --command=bash runtime/sm.puri.Sdk/x86_64/42
gsettings set org.gtk.Settings.Purism is-phone true

We need to allow the sandboxed version of gtk3 to access this property, by patching https://gitlab.gnome.org/GNOME/xdg-desktop-portal-gnome/

CC: @alexander.mikhaylenko