- 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.
* fix error about pop
* location-history: add optional callbacks
* set handler on first tick
discovered by @dcposch
* Show error when media format is unsupported
Before this change, the player would just get stuck on the loading
screen forever without notifying the user.
