Throw if set_child is called for default arrow button

libs/ui/source/default_element_factory.cpp

@@ -581,6 +581,11 @@ namespace psemek::ui
 				: direction_{direction}
 			{}
 
+			std::shared_ptr<element> set_child(std::shared_ptr<element>) override
+			{
+				throw std::runtime_error("default arrow button doesn't support children");
+			}
+
 			geom::box<float, 2> size_constraints() const override
 			{
 				auto b = button::size_constraints();