Fix regression that no update to Page.fragment_node.
Make UIEvent.view be in a Cell.
Make Document.implementation be in a Cell.
Remove the implementation OptionalSettable for Option<JS<T>> to remove needless '&mut self'.
Remove needless lifetime parameter from LayoutNodeHelpers.
auto merge of #2543 : Ms2ger/servo/nyi, r=jdm

These stub implementations bring no value, and significantly increased
maintenance costs.

This commit also synchronizes the IDL with the HTML specification and
makes the URLs consistent.
Remove needless '&mut self' from node.rs.
Remove HTMLMainElement.

https://github.com/mozilla/servo/issues/2541
auto merge of #2544 : saneyuki/servo/rm_main, r=jdm

Fix #2541
auto merge of #2535 : saneyuki/servo/mut, r=jdm

Related #2514
Remove not-yet-implemented APIs, and update MouseEvent to specification.