This will make setting the playtime for the engine easier since
the current savestate stores it as a string.
This value gets set at the same time that the string playtime gets set.
Now SaveStateDescriptor no longer subclasses HashMap. Instead all possible
saved meta data is included directly into SaveStateDescriptor.
This is slightly less flexible, but we never needed that flexibility so far.
On the other hand it should reduce the memory usage. At least on my system
(Linux/amd64) the old SaveStateDescriptor had a size of 928 and the new
SaveStateDescriptor has a size of 200.
