Feature: allow the use of TURN to connect client and server together

TURN is a last resort, used only if all other methods failed.
TURN is a relay approach to connect client and server together, where
openttd.org (by default) is the middleman.

It is very unlikely either the client or server cannot connect to
the STUN server, as they are both already connected to the Game
Coordinator. But in the odd case it does fail, estabilishing the
connection fails without any further possibility to recover.

src/window_type.h

@@ -478,6 +478,12 @@ enum WindowClass {
 	 */
 	WC_NETWORK_STATUS_WINDOW,
 
+	/**
+	 * Network ask relay window; %Window numbers:
+	 *   - 0 - #NetworkAskRelayWidgets
+	 */
+	WC_NETWORK_ASK_RELAY,
+
 	/**
 	 * Chatbox; %Window numbers:
 	 *   - #DestType = #NetWorkChatWidgets