[IE] Source dialog fires beforeunload event

[Karen Ananiev]

1. Attach the "onbeforeunload" event to the page (I used repleacebyclass.html from samples folder):

<script>
	window.attachEvent('onbeforeunload', function() {
		return 'You will lose your content.';
	});
</script>

2. Add "sourcedialog" plugin.

3. Open editor, open source dialog and click OK. The confirm dialog will appear.

[Jakub Ś]

This issue is a duplicate of #9958.