Bug 121939 [Viewers] TreeSelection should subclass StructuredSelection
TreeViewer.getSelection() now returns a TreeSelection, which implements IStructuredSelection, but does not extend StructuredSelection. Clients who downcast to StructuredSelection instead of IStructuredSelection will be broken unless we make TreeSelection a subclass of StructuredSelection.
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/TreeSelection.java
* (non-Javadoc)
	 * 
	 * @see org.eclipse.jface.viewers.IStructuredSelection#size()
	 */
	public int size() {
		if (paths != null) {
			return paths.length;
		}
		return 0;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.jface.viewers.IStructuredSelection#toArray()
	 */
	public Object[] toArray() {
		int size = size();
		Object[] selection = new Object[size];
		for (int i = 0; i < size; i++) {
			selection[i] = paths[i].getLastSegment();
		}
		return selection;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.jface.viewers.IStructuredSelection#toList()
	 */
	public List toList() {
		int size = size();