Add e.returnValue in addEventListener('beforeunload') for sake of Firefox

git-svn-id: http://svg-edit.googlecode.com/svn/trunk@2680 eee81c28-f429-11dd-99c0-75d572ba1ddd

editor/svg-editor.js

@@ -4578,6 +4578,7 @@
 				// showSaveWarning is set to 'false' when the page is saved.
 				if (!curConfig.no_save_warning && Editor.showSaveWarning) {
 					// Browser already asks question about closing the page
+					e.returnValue = uiStrings.notification.unsavedChanges; // Firefox needs this when beforeunload set by addEventListener (even though message is not used)
 					return uiStrings.notification.unsavedChanges;
 				}
 			}, false);