Feature: opt-in survey when exiting a game

On first start-up, the game will ask if you want to participate
in our automated survey. You have to opt-in, and can easily opt-out
(via the Options) at any time.

When opt-in, whenever you exit a game, a JSON blob will be send
to the survey server hosted by OpenTTD. This JSON blob contains
information that gives a global picture of the game just played:
- What settings were used
- How many humans vs AIs
- How long the game has been played
- Basic information about the OS / CPU

All this information is kept very generic, so there is no
chance we send private information to our survey server.
Nothing in the JSON blob could identify you as a person; it
mostly tells about the game played. At any time you can see
what the JSON blob includes, by pressing the "Preview Survey
Results" button in-game.

src/window_type.h

@@ -483,6 +483,12 @@ enum WindowClass {
 	 */
 	WC_NETWORK_ASK_RELAY,
 
+	/**
+	 * Network ask survey window; %Window numbers:
+	 *  - 0 - #NetworkAskSurveyWidgets
+	 */
+	WC_NETWORK_ASK_SURVEY,
+
 	/**
 	 * Chatbox; %Window numbers:
 	 *   - #DestType = #NetWorkChatWidgets