add saving and loading support

[FrenkelS]

Implement saving and loading of games.

[FrenkelS]

The Save Game menu option has been disabled, but disabling the Load Game menu option results in compile errors somehow.

[giobbino]

I'm adding myself to the request of the SAVE and LOAD options. The game run very smoothly on a 386sx (and maybe on a 286, I didn't try it yet), so it's really fun to play, but without the chance to save and load the game, it can't be actually played, unless you want to start every time from the level 1.

[FrenkelS]

In the mean time, the IDCLEV cheat can be used to warp to the next level.

[giobbino]

great!