Add an InputDeviceMapping class

It holds the mapping table device event -> retro device id, among other
things.
InputManager: add an enumerateDevices() slotted method
Move InputManager to a global variable

Also, add a Q_INIT_RESOURCE to the SDLEvents constructor, since it will
now be called before app.exec()
Joystick: Temporary fix to use the first joystick detected

Now that we moved SDLEvents instance to a global, it's initialized very
early and Joystick can't catch the DEVICEADDED event.
UI: Show the input device list on the Settings window