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Add FilteredListWalker: a filtered view over a list walker (#1593)

* Add FilteredListWalker: a filtered view over a list walker

A read-only list walker that shows the items of a source list walker passing a predicate.
Its positions are the source indices, so no mapping between two index spaces is needed.

- The source is a list or deque walker; any other iterable is copied into a SimpleFocusListWalker.
- predicate, set_predicate() and refilter() re-apply the filter; every source change does too.
- The focus stays on its widget while visible, and returns to it when it becomes visible again.
- A hidden focus moves to the next visible item after a source change, to the first after a predicate change.
- A rebuild walks the source once, and a predicate that raises leaves the view unchanged.
- The view holds the source connection weakly, so a dropped view is freed while the source lives.
- examples/filter_list.py narrows a list as a query is typed, keeping the cursor in the query.

* Empty a FilteredListWalker when the predicate raises on a source change

After a change to the source the previous visible positions may no longer exist there,
so keeping them let get_focus() and a ListBox read the wrong widget or raise IndexError.

- The view empties itself, emits "modified", and re-raises to the code that changed the source.
- It keeps the widget focused last, so the next change to the source or the predicate returns to it.
- Only the predicate is guarded: a listener of the view that raises leaves the rebuilt view in place.

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