* 1. Bug phenomenon: Only in MacOS, the Message Dialog originating from the command-line context fails to dismissing itself posterior to click the button "OK".
2. Solution: If the parent-window handle is absent during the initialization of the MessageDialog instance, the ABI `CFUserNotificationDisplayAlert` will be invoked to pop up a message dialog, instead of instantiate a NSAlert instance.
3. Ramification: Only message dialog for MacOS
4. Unit Test: cover both sync and async message dialogs and run the unit test by `cargo run --example msg`

* 删除无用的 pub 可见性标记

* @PolyMeilex is right. Accordingly, I made the following enhancements:
1. Remove the conditional dependency on the `futures` crate.
2. Relinquish the heavy thread-pool instrument.
3. Spawn a background thread by means of the std lib.

* delete the temporary variable `win` by moving `self.parent.as_ref()` into `ModalFuture::new(..)`

* Add a new conditional branch for awaiting timeout.

…ws (#211) (#224)

fix #221