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/gfx.cpp

@@ -47,6 +47,7 @@ bool _screen_disable_anim = false;   ///< Disable palette animation (important f
 std::atomic<bool> _exit_game;
 GameMode _game_mode;
 SwitchMode _switch_mode;  ///< The next mainloop command.
+std::chrono::steady_clock::time_point _switch_mode_time; ///< The time when the switch mode was requested.
 PauseMode _pause_mode;
 Palette _cur_palette;