Add hidden preference to show deprecation messages in chat area

In 1.15 the preference will be added to the advanced preferences pane.

data/advanced_preferences.cfg

@@ -185,6 +185,15 @@
 [/advanced_preference]
 
 #ifdef __UNUSED__
+[advanced_preference]
+    field=show_deprecation
+    # TODO: Add translation marks and enable this after 1.14
+    name="Show deprecation messages in-game"
+    description="Show warnings about deprecated API in the in-game chat area. These messages will still be shown in the log, even if this is disabled. In addition, the deprecation log-domain controls how many messages are printed."
+    type=boolean
+    default=no
+[/advanced_preference]
+
 [advanced_preference]
     field=joystick_support_enabled
     name= _ "Joystick support"

src/deprecation.cpp

@@ -19,6 +19,7 @@
 #include "tstring.hpp"
 #include "game_config.hpp"
 #include "version.hpp"
+#include "preferences/general.hpp"
 
 // Set the default severity with the second parameter.
 // -1 means the default is to never log on this domain.
@@ -61,11 +62,12 @@ std::string deprecated_message(const std::string& elem_name, int level, const ve
 		message += "\n  ";
 		message += detail;
 	}
-	// TODO: Decide when to dump to wml_error()
 	if(log_ptr && !log_ptr->dont_log(log_deprecate)) {
 		const lg::logger& out_log = *log_ptr;
 		out_log(log_deprecate) << message << '\n';
-		//lg::wml_error() << message << '\n';
+		if(preferences::get("show_deprecation", false)) {
+			lg::wml_error() << message << '\n';
+		}
 	}
 	return message;
 }