Add Gamepad Support

[kblaschke]

Since the application uses SDL2 and the new UI also supports gamepad input, it's just logical to suport gamepad (and joystick) input to control different functions and parts of the UI. Here's my personal proposal for button bindings, but since I'm not the gamepad gamer type, other suggestions are really welcome:

• Next preset immediate: RB
• Next preset slow transition: RT
• Previous preset immediate: LB
• Previous preset slow transition: LT
• Back (last preset): B
• Lock/unlock preset: A
• Toggle Shuffle: Y
• Select audio device: L/R
• Change beat sensitivity: U/D
• Toggle UI: Start

[Stevelande]

• Select+D-Pad right = Switch to next Monitor
• Select+D-Pad Up = Quit
• Select+D-Pad left = fullscreen on/off