LibGUI: Expose GUI::Button's "default" property to GML

Userland/Libraries/LibGUI/Button.cpp

@@ -43,6 +43,7 @@ Button::Button(String text)
         { Gfx::ButtonStyle::Coolbar, "Coolbar" });
 
     REGISTER_STRING_PROPERTY("icon", icon, set_icon_from_path);
+    REGISTER_BOOL_PROPERTY("default", is_default, set_default);
 }
 
 Button::~Button()