Make RenderChan a newtype struct.

Also, derive Clone rather than implementing it manually.
Clean up AttributeHandlers::get_attribute() for JSRef.
auto merge of #2489 : zwarich/servo/render-chan-newtype, r=jdm

Also, derive Clone rather than implementing it manually.
auto merge of #2491 : saneyuki/servo/attr, r=jdm
Add stubs for inline event handler manipulation.
Implement window forwarding for body event handlers.
Compile JS strings when setting inline event handlers via element attributes.
Port modern callback handling code from Gecko, and copy related WebIDL parser bits too.