fix #607 PageEngine containerId constraints

Container ID can't be body because we need to temporarily clone the container to perform animation.
Hence this would
1) violate HTML spec (two BODY elements in the page)
2) in the real world scenario (tested in Firefox and Chrome), when we switch to the "alternative BODY", no AT event listeners are working any longer, since they were registered on the "original BODY". It's better not to mess with that probably.