Bug 659022 - gtk_tree_model_filter_clear_cache_helper: assertion failed

Bug 659022 - gtk_tree_model_filter_clear_cache_helper: assertion failed

This bug is resolved by fixing two things in
gtk_tree_model_filter_row_deleted():

  (1) It is possible for an elt to have elt->visible_siter == NULL, when
  it is deleted.  Only call g_sequence_remove() if this pointer is
  non-NULL.

  (2) For the case len (level->seq) > 1, free the elt->children level
  if non-NULL.  Failing to do this means the level will stick around.
  If this child level was not referenced, it will still have a zero
  ref count on its parent which cannot be removed!

For both bugs unit tests have been added in the preceding commit.