Navigate filtered nodes with arrow keys + smaller cleanup

[eibx]

I've added the option to use up and down arrows to select the previous and next filter matches.

I've also removed code relating to _tree_filter_typing.

As far as I can see, _tree_filter_typing and _tree_filter_visible are always in sync (always both true or false).

And because of this an early exit in on_key:

if not self._tree_filter_visible:
    # Pass to next mixin in chain (e.g., AutocompleteMixin)
    super().on_key(event)  # type: ignore[misc]
    return

I can't see how any of the not self.tree_filter_typing statement could be reached.

Again thanks for your work 👍