Add the capability to display overlay images over node and state images

To display overlay images, do the following:
- Add overlay images into the Images and StateImages image collections.
- Assign a node’s OverlayIndex and OverlayStateIndex properties. At design time, you can assign these properties via the Items Editor.
- To dynamically specify overlay image indexes, handle the TreeList control’s OnGetNodeImageIndex event. Within an OnGetNodeImageIndex event handler, check the AIndexType parameter for tlitOverlayIndex and tlitOverlayStateIndex values and pass overlay image indexes via the AIndex parameter.