Improve resource load cache

-Added a new method in Resource: reset_state , used for reloading the same resource from disk
-Added a new cache mode "replace" in ResourceLoader, which reuses existing loaded sub-resources but resets their data from disk (or replaces them if they chaged type)
-Because the correct sub-resource paths are always loaded now, this fixes bugs with subresource folding or subresource ordering when saving.

modules/visual_script/visual_script_expression.cpp

@@ -1506,6 +1506,19 @@ VisualScriptNodeInstance *VisualScriptExpression::instance(VisualScriptInstance
 	return instance;
 }
 
+void VisualScriptExpression::reset_state() {
+	if (nodes) {
+		memdelete(nodes);
+		nodes = nullptr;
+		root = nullptr;
+	}
+
+	error_str = String();
+	error_set = false;
+	str_ofs = 0;
+	inputs.clear();
+}
+
 VisualScriptExpression::VisualScriptExpression() {
 }