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/openttd.h

@@ -11,6 +11,7 @@
 #define OPENTTD_H
 
 #include <atomic>
+#include <chrono>
 #include "core/enum_type.hpp"
 
 /** Mode which defines the state of the game. */
@@ -53,6 +54,7 @@ enum DisplayOptions {
 
 extern GameMode _game_mode;
 extern SwitchMode _switch_mode;
+extern std::chrono::steady_clock::time_point _switch_mode_time;
 extern std::atomic<bool> _exit_game;
 extern bool _save_config;
 
@@ -86,6 +88,7 @@ void HandleExitGameRequest();
 void SwitchToMode(SwitchMode new_mode);
 
 bool RequestNewGRFScan(struct NewGRFScanCallback *callback = nullptr);
+void GenerateSavegameId();
 
 void OpenBrowser(const char *url);
 void ChangeAutosaveFrequency(bool reset);