def hook(callback):
_listener.add_handler(callback)<EOL>return callback<EOL>
Installs a global listener on all available mouses, invoking `callback`
each time it is moved, a key status changes or the wheel is spun. A mouse
event is passed as argument, with type either `mouse.ButtonEvent`,
`mouse.WheelEvent` or `mouse.MoveEvent`.

Returns the given callback for easier development.