GtkExpander arrow is not dimmed when the Expander is not :sensitive

I noticed this when trying to figure out why I have no sound: The Advanced expander is insensitive because there's nothing to do with the Dummy Output, but the arrow is fully intense, which contradicts the label by looking clickable:

There don't seem to be any "base states" selectors generic enough to catch this, so I think we'll just need some copy-pastad rules for the expander title > arrow. (Note that -gtk-icon-effect does nothing for symbolic icons.)

This applies to both Adwaita and HighContrast.