Cleanup ClientLauncher structure (#10160)

Remove duplicated variables and unify the startup data into a new (inherited) struct.

src/gameparams.h

@@ -23,10 +23,29 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 
 struct SubgameSpec;
 
+// Information provided from "main"
 struct GameParams
 {
+	GameParams() = default;
+
 	u16 socket_port;
 	std::string world_path;
 	SubgameSpec game_spec;
 	bool is_dedicated_server;
 };
+
+// Information processed by main menu
+struct GameStartData : GameParams
+{
+	GameStartData() = default;
+
+	bool isSinglePlayer() const { return address.empty() && !local_server; }
+
+	std::string name;
+	std::string password;
+	std::string address;
+	bool local_server;
+
+	// "world_path" must be kept in sync!
+	WorldSpec world_spec;
+};