Improve TreeView virtualization on Windows

Currently TreeView calls getVisibleCell() to return a cell, but this doesn't work if the cell is not visible. This breaks accessibility on Windows.

This jira issue will track the improvement of this problem. Two possible ways to improve are:

1) To partially devirtualize the TreeView (we'd need to devirtualize all nodes of expanded tree items as well as the tree item itself, up to the root node).
2) We could explore the Windows virtualization APIs, e.g.:

ItemContainer:
http://msdn.microsoft.com/en-us/library/windows/desktop/ee671280%28v=vs.85%29.aspx
VirtualizedItem:
http://msdn.microsoft.com/en-us/library/windows/desktop/ee684094%28v=vs.85%29.aspx