Cocoa: Fix opening dialogs from another thread

[jay3d]

This fix will prevent crashes with NSWindow when trying to open dialogs from another thread.

[btzy]

Thanks for the PR! This has been raised before in #46, and I kind of think that it is the responsibility of the user to call the file dialog on the main thread. Both macOS and Linux/GTK require the dialog to be called from the main thread, and it is generally recommended to keep all UI work on the main thread. It also requires that the main thread is not blocked by anything else, so it might cause surprising behaviour for downstream consumers if that isn't the case.

If Java's threading system is really an issue for many people, maybe we could have a flag to explicitly opt into this just for the purpose of building a Java library, but otherwise I do not think we should encourage this use case.