Fix exceptions in the Cities log

[kaenganxt]

• Use customized, stripped down version of protobuf-net which has no further assembly dependencies
• Only put correct dlls to output directory so we can use the whole directory for an installation
• Fix exception about our save file by adding an empty package pathpath
• Correctly cancel the rich presence thread
• Shutdown our own Steam integration when Cities shuts down to prevent crashes
• Check for the existence of the simulation manager when shutting down the game as client
• Simplify the call to OpenNAT

This should resolve all exceptions in the Cities log which where caused by us.

Contributes to #289