Codechange: move all NetworkGameInfo related functions to a single file

It currently was a bit scattered over the place. Part of
NetworkGameInfo is also the GRF Identifiers that goes with it.

src/network/network_internal.h

@@ -94,8 +94,6 @@ void NetworkAddServer(const char *b);
 void NetworkRebuildHostList();
 void UpdateNetworkGameWindow();
 
-bool IsNetworkCompatibleVersion(const char *version);
-
 /* From network_command.cpp */
 /**
  * Everything we need to know about a command to be able to execute it.