Start implementing notifications:
	- resourceEntered
	- resourceLeft
Configurable in
	<script for="hf-frameset">
	({
	resourceEntered: function(msg) {},
	resourceLeft: function(msg) {}
	})
	</script>
Replace all <hf-frame>'s with placeholders **before** creating HFrameDefinition
(or just validity check in the case of frame declarations).