void render(Treeitem item,
Renders the data to the specified tree item.
item - the Treeitem to render the result.
When this method is called, the treeitem should have no child
at all, unless you don't return
Treeitem and Treerow are only components that allowed to be
A new treerow should be constructed and append to item, when
treerow of item is null. Otherwise, when treerow of item is not null,
modify the content of the treerow or detach the treerow's children first, since that only one treerow is allowed
Do not append any treechildren to item in this method, a treechildren will be appended afterward.
When a treerow is not appended to item, generally label of item is displayed.
data - that is used to render the Treeitem
index - the index of the data that is currently being rendered.
Notice the index is the order of the siblings (i.e., data that belongs
to the same parent).