Meta bug to track multi window support:

• Support multiple windows #13338
• [multi window] Constellation should not assume there's one and only one compositor #13993
• [multi window] Constellation should be able to handle multiple root frame #13994
• WebViewBuilder should take a RenderingContext to specify where it renders  #13995
• Multiwindow example is broken on OS X rust-windowing/winit#56
• Add support in servoshell for multiple top-level windows
  • servoshell: Move Minibrowser to headed window #40792
  • servoshell: Add architectural support for opening multiple windows #40883
  • libservo: Starting removing the multi-document interface API #40923
  • servoshell: Add support for opening new windows on desktop #40927