Possible layouts for an overlay.
| HorizontalToolbar | Contents will be sized to fit within a toolbar and arranged horizontally. | 
| VerticalToolbar | Contents will be sized to fit within a toolbar and arranged vertically. | 
| Panel | Contents will be presented as a normal overlay with a header title and content area. | 
| All | Equivalent to Layout.Panel | Layout.HorizontalToolbar | Layout.VerticalToolbar. |