Slightly unexpectedly, `g_icon_serialize()` doesn’t produce a floating
`GVariant`, it produces one with full ownership and returns that. That’s
not the convention for `GVariant` return values from functions which
build variants, but there’s nothing we can do to change this now as that
would be an API break.

Signed-off-by: Philip Withnall <withnall@endlessm.com>