variant: Fix double-free

gio::Settings::set_value takes over ownership of the Variant sometimes, but in other cases it doesn't. To prevent this being a problem, the custom Variant is made of the type that will never have its ownership taken.

This is not necessarily consistent with what gtk-rs authors intended.

In practice, the ownership is shared by refcounting, and after the Rust reference is dropped, one taken by Settings survives.