This widget has a special relationship with WDS_HierarchyNode.
WDS_HierarchyForm is generally only instantiated by
WDS_HierarchyNode, as a direct data member.
Together they allow dynamic building of hierarchial lists.
Even so, WDS_HierarchyForm allows children of any type.
These other children fall to the bottom of the list.
void DeleteChildren(long number)
Remove the specified number of children from the bottom of the list.
If number<0, all children are removed.
Children who are idenitifed as type WDS_HierarchyNode are deleted;
others are just removed.
This function is generally only used internally.