Add focus functions for scroll up, down, left, right

[pspeed42]

We have standard InputMapper functions for things like next, previous, and so on. This was approach 2 in issue #44 as originally written. I'm breaking it out into a separate task.

This issue would be to add similar functions for different scroll directions and then default-wire them to things like the right joystick, mouse wheel, and maybe pageup/pagedn keys or something.

ListBox would be the test bed for these new functions as it's the most obvious case where this would be useful.