Allow dragging custom node types onto array property editor

For example, an Array[MyScript] property now accepts any node with the
script MyScript on it. This works for Node and NodePath arrays alike,
and also with the @export_node_path annotation.

editor/editor_properties_array_dict.cpp

@@ -614,7 +614,8 @@ bool EditorPropertyArray::_is_drop_valid(const Dictionary &p_drag_data) const {
 			ERR_FAIL_NULL_V_MSG(dropped_node, false, "Could not get the dropped node by its path.");
 
 			if (allowed_type != "NodePath") {
-				if (!ClassDB::is_parent_class(dropped_node->get_class_name(), allowed_type)) {
+				if (!ClassDB::is_parent_class(dropped_node->get_class_name(), allowed_type) &&
+						!EditorNode::get_singleton()->is_object_of_custom_type(dropped_node, allowed_type)) {
 					// Fail if one of the nodes is not of allowed type.
 					return false;
 				}
@@ -625,7 +626,8 @@ bool EditorPropertyArray::_is_drop_valid(const Dictionary &p_drag_data) const {
 				if (!allowed_subtype_array.has(dropped_node->get_class_name())) {
 					// The dropped node type was not found in the allowed subtype array, we must check if it inherits one of them.
 					for (const String &ast : allowed_subtype_array) {
-						if (ClassDB::is_parent_class(dropped_node->get_class_name(), ast)) {
+						if (ClassDB::is_parent_class(dropped_node->get_class_name(), ast) ||
+								EditorNode::get_singleton()->is_object_of_custom_type(dropped_node, ast)) {
 							is_drop_allowed = true;
 							break;
 						} else {