Make `Window` `!Send + !Sync`

Part of https://github.com/rust-windowing/winit/issues/3367, opening to discuss separately.

We've worked hard on trying to make Window safely Send + Sync, but this somewhat obscures what's really happening underneath the hood: Some backends use Mutex, others like macOS/iOS dispatch the events to the main thread (and you may even deadlock if you try to use the window from a thread while the event loop is not running).

So maybe this is actually the wrong solution, and windows should not actually be Send + Sync? Though if we remove it, we'll still have to find some way of allowing users to render on different threads.

Are there other use-cases for Send + Sync windows (other than rendering)?

Related: https://github.com/rust-windowing/winit/issues/3317, https://github.com/rust-windowing/winit/issues/3434.

Start by reading the discussions in issues #3367, #3317, and #3434, then examine how the current Window threading guarantees interact with backend event dispatch. Done would be a resolved design direction for the Send/Sync decision and a clear account of how rendering on other threads should work.