Basic gamepad GUI control (WIP)

[appgurueu]

A starting point for using GUIs from a gamepad. Dpad directionally changes focus, A button is a press, B button closes menus, shoulder buttons change active tab.

To do

This PR is a Work in Progress.

• Maybe: Settings for all of these. Though I think it's fine to leave them hardcoded, at least in a first draft, just as their keyboard counterparts are; the gamepad buttons are standardized.
• Maybe: Axis instead of dpad? This is what STK does. I think the discrete dpad makes a lot of sense, though.
• Come up with some scheme for trickier elements, in particular inventory lists.

How to test

Pick up a gamepad, navigate some menus.