Previously navigatePrev() was changing indexh and indexv; instead should
call slide(h, v) and let it change indexh and indexv.

They looked a little wonky if you used a G or a Y.

update serif (controls, attribution URL, and fix <a> elements)

The Serif theme was a little flaky previously

This way, authors can use HTML for special formatting.

remotes plugin more natural bindings: tap — next, zoom-out — activate overview, zoom-in — deactivate overview