Memory Editor: Enable navigation with gamepad

Thank you. I might want to handle that more generally along with #43 so scrolling and pageup/down/home/end gets supported as well, but it's surprisingly not trivial (mostly because memory editor code is so old now).

Aren't you bothered by the fact that other gamepad functions are not available there?

Aren't you bothered by the fact that other gamepad functions are not available there?

Not really. I mean I don't even know how they are supposed to work.

I added D-pad nav locally because I was trying to use it on the 3DS, which does not have a keyboard, so all I have are the gamepad and the tiny touchscreen. It is hard to scroll precisely with the scrollbar, so the only other option is the D-pad.