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/widget.cpp

@@ -1172,7 +1172,7 @@ NWidgetCore::NWidgetCore(WidgetType tp, Colours colour, uint fill_x, uint fill_y
 	this->widget_data = widget_data;
 	this->tool_tip = tool_tip;
 	this->scrollbar_index = -1;
-	this->text_colour = TC_BLACK;
+	this->text_colour = tp == WWT_CAPTION ? TC_WHITE : TC_BLACK;
 	this->text_size = FS_NORMAL;
 	this->align = SA_CENTER;
 }