Add keyboard nav to project viewer in browser

In the browser I am used to using my cursor keys for step-wise navigation (typically "prev" vs "next" navication, but sometimes omnidirectional), and it feels like our play mode is a perfect candidate for this, too.

cursor key -> nagivate one page in that direction, rather than clicking the dpad buttons. This should be relatively little work to add to the dpad, such that every time it renders a directional tab, it also adds a document listener for keyup for the relevant key, unbinding it again when the tab is removed from the rendered content.