[GenericModel] Workaround a bug in python-gi 3.8

Recent python GI wrappers have a bug where Gtk.TreeIter.userdata
cannot hold value 0 (it will become None). Workaround by setting
'value+1' and getting 'value-1'.

See https://bugzilla.gnome.org/show_bug.cgi?id=698366 for the
python-gi bug.

Fixes #36.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>