Add NULL and None checking to Model.set_filter
Add NULL and None checking to Model.set_filter

If the func argument is None, pass NULL to the set_filter method to
disable the filter. The same goes for set_sort.
Also check for NULL pointers in optional py_data.

GNOME/pyclutter,pmarti/pyclutter,GNOME/pyclutter,pmarti/pyclutter,pmarti/pyclutter,pmarti/pyclutter