De @-mut windowing.

I'm submitting a part of this at first to get your feedback on whether I'm removing them in the style you like (before I go and push this strategy through the rest of non-script Servo). Note that Windowing is a little weird because its calls bounce down into C code which then re-calls into Rust, getting a handle on the Rc'd Window object out of TLS.

r? @pcwalton