Override GLib.Source.__new__() function in INOTIFY handlers

- gi.repository.GLib.Source class recently added own version of __new__()
  function

- __new__() function is called with same arguments as child's __init__()
  if child doesn't provide own version of __new__() function

- closes rhbz#908712

Signed-off-by: Jakub Filak <jfilak@redhat.com>