Allow assigning joysticks to a port

[thrust26]

Currently the ports are assigned automatically, based on joystick IDs generated by SDL. It would be better if the player could select which joystick should control which port.

This was first discussed in #494.

[thrust26]

• The JoystickDialog should be updated on-the-fly when a controller is (un)plugged (feasible?).
• The JoystickDialog has to be extended to allow selecting a port or auto (default) for each controller.
• The port has to be stored in the StickDatabase for each controller.
• The event mapping in PhysicalJoystickHandler has to be based on the port of the mapped controller. In case of auto the port will be mapped automatically (similar to current mapping).