Implement string representations for SuperflatSettings

[skatrak]

In the server.properties file, the superflat settings are specified differently than the internal representation of the program.
The following methods must be implemented:

• setStringRepresentation() in superflatsettings.cpp. This has to set the object's contents to the contents specified by the string representation.
• getStringRepresentation() in superflatsettings.cpp. This must let us recover the string representation of the object to be able to write it in a server.properties file.

The string representation of the object must follow the format specified here.