Refactor browser unload task handling

Make it possible to register multiple task functions that will be
called when the window is unloaded

core/modules/saver-handler.js

@@ -75,14 +75,14 @@ function SaverHandler(options) {
 			}
 		});
 		// Set up our beforeunload handler
-		window.onbeforeunload = function(event) {
+		$tw.addUnloadTask(function(event) {
 			var confirmationMessage;
 			if(self.isDirty()) {
 				confirmationMessage = $tw.language.getString("UnsavedChangesWarning");
 				event.returnValue = confirmationMessage; // Gecko
 			}
 			return confirmationMessage;
-		};
+		});
 	}
 	// Install the save action handlers
 	if($tw.browser) {