Create an interface for customizing the game controller button mappings

[broadwell]

This depends on issue #158 already being addressed.

It probably makes sense to reuse some of the functionality and UX elements already implemented for remapping the QWERTY keyboard mappings (accessible from the "gear" settings menu of the UI and clicking the "Configure Key Bindings" button) to build the mapping UX for the gamepad.

Fortunately there's a very standardized set of on/off buttons and continuous input values for modern gamepads, although it may get kind of tricky to work out how to empower the user to map these to various aspects of the pianolatron controls.