Saving the game should save all temporary (touched/dirty) maps

[darkf]

In order for a savegame to be complete it needs to save every map loaded/touched from the base game.

This means keeping around a dict of {map: SerializedMap} in memory and switching to the serialized variant instead of loadMap when loading back to it. (We really probably need some kind of Game manager class that handles the loading/switching of maps.)