Allow passing a method, request body, and headers to the pipeline in LoadUrlMsg
auto merge of #3626 : Manishearth/servo/form-prep, r=jdm

Framework for form submission
Disable failing reftest (#3636)
auto merge of #3637 : mbrubeck/servo/disable-link-style-reftest, r=pcwalton
Move windowing code out of the compositor

This is mainly just moving code around, in preparation for further changes to
the "windowing" API.