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.

scene/resources/theme.cpp

@@ -930,6 +930,9 @@ void Theme::get_type_list(List<StringName> *p_list) const {
 	}
 }
 
+void Theme::reset_state() {
+	clear();
+}
 void Theme::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_icon", "name", "node_type", "texture"), &Theme::set_icon);
 	ClassDB::bind_method(D_METHOD("get_icon", "name", "node_type"), &Theme::get_icon);