Add joystick simulation

[sy2002]

The ZX-Uno for MEGA65 core has a very nice feature: If you switch on Caps Lock then the cursor keys of the MEGA65 in conjunction with the Space bar are simulating a joystick.

For users of our cores, who don't have a joystick handy this might be a game changer.

We might implement this feature in a way, that the simulated joystick by default is in port #1 and if one uses the "flip joystick ports" signal, then the simulated joystick goes to port #2.