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/saveload/saveload.h

@@ -353,6 +353,7 @@ enum SaveLoadVersion : uint16 {
 	SLV_EXTEND_VEHICLE_RANDOM,              ///< 310  PR#10701 Extend vehicle random bits.
 	SLV_EXTEND_ENTITY_MAPPING,              ///< 311  PR#10672 Extend entity mapping range.
 	SLV_DISASTER_VEH_STATE,                 ///< 312  PR#10798 Explicit storage of disaster vehicle state.
+	SLV_SAVEGAME_ID,                        ///< 313  PR#10719 Add an unique ID to every savegame (used to deduplicate surveys).
 
 	SL_MAX_VERSION,                         ///< Highest possible saveload version
 };