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/widgets/network_content_widget.h

@@ -36,7 +36,7 @@ enum NetworkContentListWidgets {
 	WID_NCL_DETAILS,        ///< Panel with content details.
 	WID_NCL_TEXTFILE,       ///< Open readme, changelog (+1) or license (+2) of a file in the content window.
 
-	WID_NCL_SELECT_ALL = WID_NCL_TEXTFILE + TFT_END, ///< 'Select all' button.
+	WID_NCL_SELECT_ALL = WID_NCL_TEXTFILE + TFT_CONTENT_END, ///< 'Select all' button.
 	WID_NCL_SELECT_UPDATE,  ///< 'Select updates' button.
 	WID_NCL_UNSELECT,       ///< 'Unselect all' button.
 	WID_NCL_OPEN_URL,       ///< 'Open url' button.