Support unpacking more Variants

[dcz-purism]

I hit a snag when trying to unpack a Variant with the signature a(ss). I ended up making some custom parsing, but it would be nice to have that out of the box. For reference, this is how I unpacked it: https://source.puri.sm/dorota.czaplejewicz/squeekboard/blob/406b4fda80f53627670db1843f3e678817411585/src/popover.rs#L87

[sdroege]

Wrong repository :) This should've been reported against the glib repository, and there we already have gtk-rs/glib#113

@GuillaumeGomez Can you close this one?