auto merge of #1751 : Ms2ger/servo/image-loading, r=jdm
Move ID management on binding to and unbinding from tree into Element.
Cleanup Element::set_attribute.
Pass a non-Option DOMString to after_remove_attr.
Call after_remove_attr before removing the attribute, and rename it before_remove_attr.
Call unregister_named_element rather update_idmap from before_remove_attr.
Only register the new ID from after_set_attr.