Transition: move hide/show into transition plugin

So that each transition can do whatever it wants.
Layout: force body to be 100% wide

Will be useful when we implement side-scrolling so the entire body isn't
scrollable in the x-dir.
Transitions: add classBased (slides-in)

Make a transition that uses css classes. Add rules that animate
the steps based on the css classes. This necessitated adding the
idea of 'direction' to showNext() and friends.