macOS/iOS main thread safety

[madsmtm]

macOS and iOS have the pervasive concept of "main thread safety", which essentially just means that every UI operation has to happen on the main thread. We have some support for this in macos::util::async, but this has been developed more on a case-by-case basis, which is not the way to do thread safety; remember, it is not just certain APIs that are thread unsafe, and the rest are fine, every API is thread unsafe unless documented otherwise!

TODO

• objc2: Allow Cell in instance variables so that we don't need to use &mut self
  • Implement Encode and RefEncode for UnsafeCell and Cell madsmtm/objc2#351
• Run winit through the XCode tools Thread Sanitizer and Main Thread Checker
• Make some mechanism so that every call on WinitWindow requires running on the main thread first.
  • Add MainThreadBound madsmtm/objc2#344
• Use the above improvements to objc2
• Move things from the global AppState/HANDLER struct into the main-thread only ApplicationDelegate.
  • macOS: Remove global HANDLER and AppState #3389
• Make MonitorHandle and Fullscreen Send + Sync.
  • Make iOS fully thread safe #3045
• Make HasWindowHandle::window_handle return an error when not on the main thread.

[madsmtm]

Pretty sure this is done now, so I'll close it, feel free to open a new issue if you encounter problems.