Introduce trigger wc_backbone_modal_before_update

To update the form data upon trigger of response event

assets/js/admin/order-backbone-modal.js

@@ -110,6 +110,8 @@
 		getFormData: function() {
 			var data = {};
 
+			$( document.body ).trigger( 'wc_backbone_modal_before_update', this._target );
+
 			$.each( $( 'form', this.$el ).serializeArray(), function( index, item ) {
 				if ( data.hasOwnProperty( item.name ) ) {
 					data[ item.name ] = $.makeArray( data[ item.name ] );