Add a warning when using plain Container, as many users seem to misunderstand what this is.

scene/gui/container.cpp

@@ -169,6 +169,19 @@ void Container::_notification(int p_what) {
 	}
 }
 
+String Container::get_configuration_warning() const {
+
+	String warning = Control::get_configuration_warning();
+
+	if (get_class() == "Container" && get_script().is_null()) {
+		if (warning != String()) {
+			warning += "\n";
+		}
+		warning += TTR("Container by itself serves no purpose unless a script configures it's children placement behavior.\nIf you dont't intend to add a script, then please use a plain 'Control' node instead.");
+	}
+	return warning;
+}
+
 void Container::_bind_methods() {
 
 	ClassDB::bind_method(D_METHOD("_sort_children"), &Container::_sort_children);

scene/gui/container.h

@@ -57,6 +57,8 @@ public:
 
 	void fit_child_in_rect(Control *p_child, const Rect2 &p_rect);
 
+	virtual String get_configuration_warning() const;
+
 	Container();
 };