We need to use the performance for the "from" route when going backwards. Also,
fix a bug in the HeroParty where it would call the quest finished callback
multiple times.

Fixes #1958

In this approach, the hero support is layered on top of the basic navigator
functionality.

We still use special-purpose typedefs in the gesture code for symmetry with
other gesture callbacks.

Fixes #1827

By default now heroes won't animate if they don't find a matching hero
on the target page. You can make them animate by setting 'alwaysAnimate:
true' on the Hero constructor.

Now you don't need to pass the navigator around everywhere.