Asynchronous file dialogs

[btzy]

It is perhaps time to revisit designing an asynchronous API.

Some considerations:

• Do we want a callback-based API (e.g. like Cocoa's native API or a polling-based API (closer to GTK/Portal's native API)?
• How can the user perform non-busy waiting?
• Should this be built into a separate static/shared library (since very few applications will want to call both sync and async functions)?

Note 1: Asynchronous file dialogs are about calling an NFD API function that shows a file dialog window, but returns before the dialog is completed. It is not about multithreading or thread-safe API calls - you are still expected to call the async functions from the main thread.

Note 2: Even after this is merged, async support will be considered unstable for a while.

Related: #64