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/network/network_gui.h

@@ -24,7 +24,8 @@ void ShowNetworkGameWindow();
 void ShowClientList();
 void ShowNetworkCompanyPasswordWindow(Window *parent);
 void ShowNetworkAskRelay(const std::string &server_connection_string, const std::string &relay_connection_string, const std::string &token);
-
+void ShowNetworkAskSurvey();
+void ShowSurveyResultTextfileWindow();
 
 /** Company information stored at the client side */
 struct NetworkCompanyInfo : NetworkCompanyStats {