Include map in save file

[jmdejong]

Right now to play a custom map save game you need both the save file and the map.
It would be better if the map is included in the save file, so the save file has all information it needs to be loaded.
This would also make more sense in the map loading; having everything in the same place.

The disadvantage would be that the savegame files would be a bit larger, but I think that would be worth it. Also, for randomly generated or built-in maps it could still only save the seed or name.

See also #2263.
I think doing this would also make it easier to solve #1552.

[LinuxDonald]

Bigger savegame are no problem.