Gio::SimpleAction: Move set_state to original pos

If I’d realised there was a specific commit that protected this, I would
have reverted it, rather than doing it manually in my previous commit.

Completes reversion of commit b7431bca –
“Gio::SimpleAction: Make set_state() protected.”

https://bugzilla.gnome.org/show_bug.cgi?id=777953