Added a common interface that:
- Uses closures instead of static functions. This should allow easier
refactoring in the future.
- Returns a handle which is invalidated on `Drop`. This should avoid
situations where the event loop has exited, but an observer is still
called because the user spawned the application later on.
- Is properly main-thread safe.
This interface is placed in winit-common, to allow using it in both
winit-appkit and winit-uikit.
