fix marshaling of arrays of GVariants

Add unit tests for marshaling of arrays of variants with all
transfer modes. Requires latest gobject-introspection.

Plug potential leaks of GArray data members

Fix calling of wrong cleanup_from_py for arrays

Simplify and fix logic for cleaning up arrays both in from_py()
and to_py() code paths.

https://bugzilla.gnome.org/show_bug.cgi?id=638915



Signed-off-by: Martin Pitt <martin.pitt@ubuntu.com>