portal: Allow forcing it off
With GTK_USE_PORTAL=1, try to use the portal even if we if we use unsupported features. With GTK_USE_PORTAL=0, never use the portal, as before. What we really want here is to detect the situation of "sandboxed without full $HOME access" and use the portal in those cases. If we make the portal smart enough to hand back direct uris for files that the app can access, we may simplify the check to just "sandboxed".
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