Add a Navigator component (for in-book navigation)

This represents a substantial equalising of navigation between
book types, and greatly increases the predictability of navigating
inside books (removing the need for Flickable items having direct
navigational impact, making swipes and taps explicit gestures).

Some thought will need to be put into making it possible to tap
on parts of books for navigation in the future, but as this is
not a feature as it is anyway, not supporting that at the moment
is less of an issue (and the way this component is implemented
means it's reasonably straightforward to work that in at a later
date)