Multiple windows in winit backend

[i509VCB]

The winit backend currently does not allow more than one window to be created.

One idea I had to make this possible is to expose raw winit::Event so you can get at WindowId. This would also mean you can use other features of winit without needing to add more code to smithay. Creating a window would also be put on downstream to do. As a result of this how winit sets up EGL would need to change (this will need to happen for the future dmabuf presentation in winit idea)