Get the dialog from a MetroWindow

I need this because i need to fill InputDialog from a barcode scanner.
This function can be used outside from the MahApps.Metro namespace.
This can be modified to not use Reflection, but it should be merged in DialogManager.cs like this
```
public static async Task<TDialog> GetDialogAsync<TDialog>(MetroWindow owner) where TDialog : BaseMetroDialog
{
    TDialog dialog = null;
    await owner.Dispatcher.BeginInvoke(new Action(() =>
    {
        if (owner.metroDialogContainer.Children.Count != 0)
            dialog = owner.metroDialogContainer.Children[0] as TDialog;
    }));
    return dialog;
}
```

Sorry for my bad english...