Make ComputedStyle fields private and add getters.

This isolates layout code from upcoming refactoring in properties.rs.mako.
Un-disable and fix some warnings in the style crate.
Remove CowArc and use the new Arc::make_unique() method instead.
Use StrBuf more efficiently in htmlserializer.
Rewrite escape() in htmlserializer to push onto a StrBuf.
auto merge of #2484 : Ms2ger/servo/htmlserializer-strbuf, 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.
auto merge of #2204 : jdm/servo/inlineev, r=Ms2ger

Fixes #1238.
auto merge of #2494 : bjz/servo/split-to-width, r=pcwalton

This will make removing the box splitting logic easier to implement. I am also using a `SplitInfo` struct for returning the split results, which should make the code more self-documenting. In the future the `TextRun` should not be returned, and `SplitInfo` should only require a single `CharIndex`, but one thing at a time...

@pcwalton r?
Submodule updates for #2137
Update submodules for #2492
auto merge of #2495 : mbrubeck/servo/2137-submodule-updates, r=larsbergstrom
Derive Clone for ChildFrameTree instead of implementing it manually.
auto merge of #2496 : zwarich/servo/deriving-clone, r=jdm
Request header and postdata support for XHR
auto merge of #2493 : Manishearth/servo/xhr-post, r=jdm

Blocks #2282
Remove unnecessary pubs from script_task