Autosave frequency option.

After losing some progress today due to a crash I decided to add an autosave frequency option, as the default interval of 5 was longer than I would prefer. ~~I also made the game save 3 times a month instead of 2 (as originally intended?).~~

Other notes regarding autosaving (not implemented):
- I think in Ironman mode game should be saved every turn or maybe even every action, but saving takes a second/two or more on slower machines so it would be annoying to some.
- Game should be automatically saved after returning to geoscape. I actually tried to implement this, but it would require creating a new state class, which I wasn't comfortable doing (forked the project today :).