Implement Cheats of original engine

[NJW1979]

I love how your port is coming along. However, due to nerve damage from a life-changing accident, I can't play JJ without the cheats and in the latest build, I can't save or load games.

[carstene1ns]

Cheats are not implemented (and I doubt will anytime soon), however, I have planned to add a debug menu to provide similar functionality.

Saving and loading is not implemented yet, since the original format is (pseudo-)encrypted.

[carstene1ns]

Savegame support is partially implemented now.
Will keep this issue to track progress with Debug Menu/Cheats.