pyg_value_from_pyobject: support GArray

This commit adds support for marshalling a python list (or other sequence)
returned from signal handlers to GArray, if necessary.

This parallels the implementation written to marshal to (the now deprecated)
GValueArray.

This fixes a crash in rhythmbox as seen downstream here:
https://bugzilla.redhat.com/show_bug.cgi?id=872851

https://bugzilla.gnome.org/show_bug.cgi?id=690514



Co-Authored-By: Martin Pitt <martinpitt@gnome.org>