Make the window be owned by the event loop

[madsmtm]

Part of #3367, opening to discuss separately.

Roughly proposed API:

let window_id = event_loop.create_window();

// Later

let window: &Window = event_loop.get_window(window_id);
window.set_title("foo");

See this comment for a full description of the issue.

Related: #3317.

[kchibisov]

In general I'd add a note that I'd like to see more ECS (entity component system) approach to the resources winit owns and then we should have weak handles or owned data types to interact with them from the code owned by the user.

In general, ECS based approach forces you to work with handles when you do complex structures, like scene trees where you want to refer to render surface. Handles allow you to freely remove or move out the resources without swap_remove, since you can just override besides being worrying for dropping the window.