Allow Game::savestats() to accept a pointer to ScreenSettings

Another step to fix the bug #556 is to allow Game::savestats() to accept
a pointer to an existing ScreenSettings struct. This entails refactoring
Game::savesettings() and Game::serializesettings() to accept the
function as well, along with adding Screen::GetSettings() so the
settings of the current Screen can be easily grabbed.

desktop_version/src/Screen.h

@@ -10,6 +10,8 @@ class Screen
 public:
 	void init(const ScreenSettings& settings);
 
+	void GetSettings(ScreenSettings* settings);
+
 	void LoadIcon();
 
 	void ResizeScreen(int x, int y);