Delegate keyboard navigation to popover

[jonathonherbert]

At the moment, the plugin manipulates the popover option index with imperative methods on the TypeaheadPopover class, which in turn passes index values to the popover renderer.

This approach means that handling more complex navigation — for example, tab navigation to swap between relative and absolute dates — couples the TypeaheadPopover (which does not know what it's rendering) with the rendered components, which will soon have opinions about what they'd like to happen when left, right and enter are pressed.

We should delegate navigation to the renderer, allowing the renderer to subscribe to navigation events, and triggering them from the plugin.