Handle GVariants for callback arguments
Handle GVariants for callback arguments

Callbacks still use GIArgument, add missing GVariant support. This is the
equivalent of what commit 9d5604220bd56 did for pygi_marshall_*().

MathieuDuponchelle/pygobject,thiblahute/pygobject,GNOME/pygobject,thiblahute/pygobject,sfeltman/pygobject,GNOME/pygobject,sfeltman/pygobject,pexip/pygobject,davibe/pygobject,jdahlin/pygobject,sfeltman/pygobject,pexip/pygobject,choeger/pygobject-cmake,pexip/pygobject,choeger/pygobject-cmake,choeger/pygobject-cmake,davibe/pygobject,GNOME/pygobject,MathieuDuponchelle/pygobject,MathieuDuponchelle/pygobject,davibe/pygobject,davidmalcolm/pygobject,davidmalcolm/pygobject,thiblahute/pygobject,jdahlin/pygobject,davidmalcolm/pygobject,jdahlin/pygobject,davibe/pygobject