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/textfile_type.h

@@ -12,13 +12,15 @@
 
 /** Additional text files accompanying Tar archives */
 enum TextfileType {
-	TFT_BEGIN,
+	TFT_CONTENT_BEGIN,
 
-	TFT_README = TFT_BEGIN, ///< NewGRF readme
-	TFT_CHANGELOG,          ///< NewGRF changelog
-	TFT_LICENSE,            ///< NewGRF license
+	TFT_README = TFT_CONTENT_BEGIN, ///< Content readme
+	TFT_CHANGELOG,                  ///< Content changelog
+	TFT_LICENSE,                    ///< Content license
 
-	TFT_END,
+	TFT_CONTENT_END, // This marker is used to generate the above three buttons in sequence by various of places in the code.
+
+	TFT_SURVEY_RESULT = TFT_CONTENT_END, ///< Survey result (preview)
 };
 DECLARE_POSTFIX_INCREMENT(TextfileType)