Add additional fire buttons to framework

[Kugelblitz360]

Currently the framework seems to support one fire button only. The DB9 standard allows for two additional buttons by using the POTX and POTY lines as digital rather than analog inputs. The C64GS used POTX, the Amiga POTY and also one shorts against GND, the other against +5V. This might need to be user switchable in the framework.

Also a clever use to emulate two further buttons is by watching the state of the joystick directions. As Up+Down can not be pressed simultaneously (as well as Left+Right), these logic states could be captured and send as a button press INSTEAD of a direction.

This would give cores the ability to use 2-, 3- or 5-button adapters.

This might require re-implementation of joystick logic in the C64 core. The core currently is working correctly (as you can test with Joyride and a Mouster), if this moves away from the core itself into the framework it might break things.