Horizontal panning

[let-def]

This is a continuation of #52 that implements horizontal panning (using left/right arrows to scroll through a page before changing).

Other ideas (discussed in the previous PR):

• support two column modes: when panning down (resp. up) and reaching a border, try to move right (resp. left) and scroll to the top (resp. bottom) to simulate navigating columns; and change page when reaching the top-left/bottom-right corners
• two columns modes should be opt-in; a simple UI could allow the user to specify the column positions for even and odd pages
• alternatively, the columns could be inferred from DVI structure