added delete button support in editor (#9053)

See https://css-tricks.com/snippets/javascript/javascript-keycodes/ for key codes.

editor/index.html

@@ -284,9 +284,11 @@
 
 				switch ( event.keyCode ) {
 
-					case 8:
+					case 8: // backspace
 						event.preventDefault(); // prevent browser back
 
+                                        case 46: // delete
+
 						var object = editor.selected;
 
 						if ( confirm( 'Delete ' + object.name + '?' ) === false ) return;