gtktreemodelfilter: rework gtk_tree_model_filter_remove_elt_from_level

gtktreemodelfilter: rework gtk_tree_model_filter_remove_elt_from_level

The most important change is that the function now properly releases
any external ref count and propagates these changes to the child model.
If a node is removed due to a filtering action, we now properly release
all external reference counts for its hierarchy of child nodes.

Apart from that, the function has been restructured to remove code
duplication.

Finally, there is still some doubt whether there are more calls to
gtk_tree_model_filter_free_level() which need unref_external set to TRUE.