'Process changes in the genre_source_field combo box
Update Excluded genres preview'

def genre_source_field_changed(self, new_index):

    new_source = self.genre_source_field.currentText()
    self.genre_source_field_name = new_source
    if (new_source != _('Tags')):
        genre_source_spec = self.genre_source_fields[unicode(new_source)]
        self.genre_source_field_name = genre_source_spec['field']
    self.exclude_genre_changed()
