Store user settings in a textual format

[mmatyas]

Because the config file is a raw binary dump, any change to the internal structures invalidates earlier config files. A textual format also enables editing the config outside the game.

Probably depends on #370.

[AzureBerylBlue]

If you do this, you should also save the player's skin selection as a string to fix the issue where adding new skins to your skin folder can change the selected skin the next time you load the game.

[mmatyas]

Now that #465 is merged, this can be done much easier.