Called when a previously-blank item becomes visible in the TreeViewer. If
the content provider knows the child element for the given parent at this
index, it should respond by calling
TreeViewer#replace(Object,int,Object). The content provider
should also update the child count for any replaced element by calling
TreeViewer#setChildCount(Object,int). If the given current child
count is already correct, setChildCount does not have to be called since
a call to replace will not change the child count. If the content
provider doesn't know the child count at this point, and can more
efficiently determine if the element has
any children, then it can
instead call
TreeViewer#setHasChildren(Object,boolean).
NOTE #updateElement(int index) can be used to determine
selection values. If TableViewer#replace(Object, int) is not called
before returning from this method, selections may have missing or stale
elements. In this situation it is suggested that the selection is asked
for again after replace() has been called.