- Handles more than 2 pages in the history robustly now!
- When self._pending is true, all navigations are ignored.
- No more bug with back() being called twice too quickly.
- Remove "leaky abstraction" methods like clearPending() and pending()
- Add backToFirst() that properly unloads each page as it goes back to
the first one.
- Enhance clearForward() to support removing a specific page from the
forward stack, instead of nuking the whole thing.
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