Enable automatic saving of each turn in battlescape

It's a bit annoying have something interesting happen, be it a crash or an anomaly,  in the battlescape and not have a save just before it happened or know how to recreate it.  I propose that we allow the option to save the game before each player and alien turn so that we are more likely to capture these events.

To avoid the introduction of yet another option I further propose that we use the existing autosave option.