The annotation applies to the method as a whole, not to individual
arguments, so the existing annotations don't have any effect.
This was hidden by gdbus-codegen's automatic handling of arguments
of type 'h', but that change was reverted, resulting in a broken
build.
GLib is already doing this, which broke a number of auto-generated code.
This tracks those changes and manually sets the annotations so that we
are guaranteed to get this behavior.
We'll need to fix other uses (such as in Mutter), but we have to do that
already because of the change in GLib.
Fixes#17
