Feature: make "join game" button join the game, instead of first showing a lobby window (#9467)

Nobody really paid attention to the lobby window, and it completely
missed its purpose. Most people don't even wait for companies to
show up, but just hit "New Company".
This in turn means people create a lot of unneeded companies, while
they "just want to watch the game" or join another company.

Instead, "Join Game" now just joins the game as spectators.

src/network/core/tcp_game.h

@@ -38,7 +38,7 @@ enum PacketGameType {
 	PACKET_CLIENT_JOIN,                  ///< The client telling the server it wants to join.
 	PACKET_SERVER_ERROR,                 ///< Server sending an error message to the client.
 
-	/* Packets used for the pre-game lobby. */
+	/* Packets used for the pre-game lobby (unused, but remain for backward/forward compatibility). */
 	PACKET_CLIENT_COMPANY_INFO,          ///< Request information about all companies.
 	PACKET_SERVER_COMPANY_INFO,          ///< Information about a single company.