Add controller support

[numpad]

The game should be as accesible as possible; Controller support is a must and will become even more important once we will implement local coop. Controller input is just as important as mouse&keyboard, maybe it will even become the preferred/recommended input method. Both Xbox and Playstation controller icons & button images are required. The correct button image should be shown even in local coop (Player1 with PS: press X to buy, Player2 with Xbox: press A to buy)

• [ ] Basic controller support
• [ ] Add Xbox specific buttons
• [ ] Add Playstation specific buttons