Change GamePad button defines to an enum

[JenniferPylko]

For the same reason that consts are preferable over defines, so are enums, debugging is easier for programmers of MINX and of MINX games.

[JenniferPylko]

This is more difficult than I thought it would be, as I can't find any documentation for how glfw maps XBox 360 buttons, plus the triggers are broken anyway in the defines. I recommend removing the defines and implementing a control scheme mechanism for 1.1