Comments VirtualizingPanel property.
Adds ListBox, ListBoxItem styles.  Renames List brushes.
Adds GroupBox styles to the Styles.xaml. Adds content to the empty menu items.
Merge commit 'c09c50eb0c3b5f4599fe52d70adb7f04bed273c9'
Adds ListBox and GroupBox to the List of supported controls to the Readme.md.