Independently control dialog show/hide animation

I find this useful to eliminate 1 sec exit delay after showing quit confirmation dialog:

![quitconfirmation](https://f.cloud.github.com/assets/740729/2438617/dfa85230-adf3-11e3-912c-1cb08d4d87c2.JPG)

Sample usage code for event:

- MainWindow.xaml
```XML
<Controls:MetroWindow 
	Closing="Window_Closing" />
<!-- Other content -->
<Controls:MetroWindow />
```

- MainWindow.xaml.cs
```C#
private bool _shutdown;

private async void MetroWindow_Closing(object sender, System.ComponentModel.CancelEventArgs e)
{
	e.Cancel = !_shutdown;
	if (_shutdown) return;

	var mySettings = new MetroDialogSettings()
	{
		AffirmativeButtonText = "Quit",
		NegativeButtonText = "Cancel",
		AnimateShow = true,
		AnimateHide = false
	};

	var result = await this.ShowMessageAsync("Quit application?",
		"Sure you want to quit application?",
		MessageDialogStyle.AffirmativeAndNegative, mySettings);

	_shutdown = result == MessageDialogResult.Affirmative;

	if (_shutdown)
		Application.Current.Shutdown();
}
```