Decouple this from winit

[icefoxen]

Basically my dream is that glutin does not need to depend on a particular version of winit, it just does the OpenGL context setup and winit's windowing and event loop and stuff is all independent of it. glutin just takes a raw-window-handle and does its magic.

Per conversations on Discord, this is Tricky 'cause on some platforms the OpenGL context setup needs to happen at window creation time, you can't create a window and then throw an OpenGL context into it later. So glutin would need to create a window and then pass its raw-window-handle to winit for it to do its magic. For my purposes this seems fine, and apparently bullet point 2 in this issue has similar problems, so this may be something we want to do anyway.

[goddessfreya]

Implemented for v0.24. See #1271. Closing.