Changed MetroDialogOptions to a DependencyProperty

This would allow me to configure MetroDialogOptions at __Style__ level:
```xml
<Style x:Key="CustomWindowsStyle"
       TargetType="{x:Type controls:MetroWindow}">
  <Style.Setters>
    <Setter Property="MetroDialogOptions">
      <Setter.Value>
        <dialogs:MetroDialogSettings ColorScheme="Accented" 
                                     NegativeButtonText="Abbrechen" />
      </Setter.Value>
    </Setter>
  </Style.Setters>
</Style>
````