Improve input mapping flexibility

[tom-seddon]

Currently you can map any PC key to any BBC key, and you have options for how PC gamepad/joysticks map to BBC joysticks. But, really, this should be more flexible, so you can map any PC thing (keyboard key, gamepad/joystick axis/button) to any BBC thing (BBC key, BBC joystick axis/button).

Main use case would be playing keyboard-only BBC games using a PC joystick/gamepad.

See also #256

[tom-seddon]

Would probably be worth doing the (now really rather overdue) dear imgui update first, due to its input API changes: #206

Might be nothing relevant involved, but I won't know until it's done.

[tom-seddon]

#245 would tie it all together nicely