New function: auto-save/resume game

I've added the function to auto-save a non-online game. The game will save at a set interval, or when you close the program while you are still in a game.
This will make it a lot easier to play the solitary mode until the end, as this mode generally can last up to several hours. It can also protect your perfect game score against unexpected crashes and such.
If your game was saved and you start the game, it will show "load previous game" in the menu. If you want to efficiently use this feature, it is advised to pause your game first, and then pick Quit from the menu, or press Alt+F4 to close the program.
In addition, I also did a bugfix on mana consumption, where magics would use MORE mana than was required for the spell. This could result in a negative MP pool that seemingly didn't regenerate for a while.

The update is available through the installer or the in-game updater.
This build is mainly for testing if the save function works as intended.