When the outline has focus, arrow keys navigate the outline instead
of changing the current page.
Add opt-up and opt-down as a way to move by one page even when the
outline has focus. (This matches Preview.app.)
xib change: Added two menu Previous Page with key equivalent opt-up
and Next Page with key equivalent opt-down to Go menu and bound them to
goToPreviousPage: and goToNextPage: on First Responder.
When the outline has focus, the responder chain is outline ->
window, so also add the actions on the window controller, and
let that forward to the PDF view.