2048: Make board size and target tile configurable

This adds a "settings" option that allows the user to configure the
board size and target tile, and optionally save them to a config file.
Closes #3219.

Games/2048/BoardView.h

@@ -49,6 +49,7 @@ private:
     size_t columns() const;
 
     void pick_font();
+    void resize();
 
     Color background_color_for_cell(u32 value);
     Color text_color_for_cell(u32 value);