Add "IsSeparatorVisible" to RibbonGroupBox

[maurosampietro]

A ribbongroupbox always shows a separator. Sometimes it is desirable to hide it.

Here's the code, let me know if you merge. Thank you.

Add a new DependencyProperty In RibbonGroupBox.cs

#region IsSeparatorVisible
public bool IsSeparatorVisible
{
    get { return (bool)GetValue( IsSeparatorVisibleProperty ); }
    set { SetValue( IsSeparatorVisibleProperty, value ); }
 }

// Using a DependencyProperty as the backing store for IsSeparatorVisible.  This enables animation, styling, binding, etc...
public static readonly DependencyProperty IsSeparatorVisibleProperty =
  DependencyProperty.Register( "IsSeparatorVisible", typeof( bool ), typeof( RibbonGroupBox ), new PropertyMetadata( true ) );
 #endregion

Add this line in File RibbonGroupBox.xaml, to element x:name="separator", line 654:

Visibility="{Binding IsSeparatorVisible, RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource BooleanToVisibilityConverter}}"