Implement commands for navigating the overlay result lists

Make it possible to map a keybinding to previous/next result in the overlay (Goto Anything etc.) results, like the up/down arrows currently do. Alternatively, make whatever is bound to "move by lines" move the "result selector" when the overlay is visible. Any one of these solutions will let those of us with the Emacs keybindings hard-wired into our brains avoid the arrow keys altogether.