Adds an icon to the specified leaf item. You have to pass the id of a Hollywood
brush/icon which should be used as the icon in br. To remove an icon from a leaf item,
pass -1 in br.
Please also read about RapaGUI's image cache to learn more about icon support in
RapaGUI. See Image cache for details.
Inputs
id
id of the leaf
column
column index of the leaf item
br
identifier of a Hollywood brush/icon to use as an icon or -1 to remove the item icon