Returns the number of child items encompassed by a given item.
Original signature is '- (NSInteger)outlineView:(NSOutlineView *)outlineView numberOfChildrenOfItem:(id)item'
Available in Mac OS X v10.0 and later.

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NSInteger OutlineViewNumberOfChildrenOfItem( NSOutlineView outlineView, Id item )
Function OutlineViewNumberOfChildrenOfItem ( _ outlineView As NSOutlineView, _ item As Id _ ) As NSInteger
NSInteger OutlineViewNumberOfChildrenOfItem( NSOutlineView^ outlineView, Id^ item )

- outlineView (NSOutlineView)
- The outline view that sent the message.
- item (Id)
- An item in the data source.

The number of child items encompassed by item. If item is nil, this method should return the number of children for the top-level item.

- Available in Monobjc Bridge: 10.6 (For Mac OS X 10.6 and later), 10.5 (For Mac OS X 10.5 and later)