Make Window !Send + !Sync

[madsmtm]

Part of #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: #3317, #3434.

[daxpedda]

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

It is still highly desirable for MT applications to be able to call all sort of Window methods in different threads.
The go-to suggestions to solve this is a WindowHandle::queue() method that basically works like the abstraction used in iOS/MacOS/Web right now: just package a closure that will be executed on the event loop.

Backends that do support certain methods to be called in any thread will have to do that via extension.

[kchibisov]

I think we should just have a WindowHandle implementing Window more or less generically with the help of the queue, however certain backends may overwrite methods they can dispatch straight away. It's a bit more work, but no need to separate functionality, etc.