Move windowing code out of the compositor

This is mainly just moving code around, in preparation for further changes to
the "windowing" API.
Update CEF port to use glfwapp
Update to latest servo/glfw-rs

Picks up servo/glfw-rs#26.
auto merge of #3533 : mbrubeck/servo/app, r=larsbergstrom

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

r? @larsbergstrom