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/intro_gui.cpp

@@ -17,6 +17,7 @@
 #include "genworld.h"
 #include "network/network_gui.h"
 #include "network/network_content.h"
+#include "network/network_survey.h"
 #include "landscape_type.h"
 #include "landscape.h"
 #include "strings_func.h"
@@ -504,7 +505,10 @@ void ShowSelectGameWindow()
 
 static void AskExitGameCallback(Window *w, bool confirmed)
 {
-	if (confirmed) _exit_game = true;
+	if (confirmed) {
+		_survey.Transmit(NetworkSurveyHandler::Reason::EXIT, true);
+		_exit_game = true;
+	}
 }
 
 void AskExitGame()