Returns the number of currently viewable items in a treeview control.
iCount = treeview.GetItemCount ([bAllItems])
|iCount||Count of currently viewable items in the treeview. INTEGER.|
|bAllItems||Optional: TRUE if count should include nonviewable items (children of non-expanded items). BOOLEAN.|
GetItemCount returns the number of currently viewable items (that is, the items with no collapsed ancestors) in the treeview, or optionally the count of all items, including those with collapsed ancestors. This is the only treeview function that can return information about nonviewable items.