New parameter in 'content_structure_tree' operator

*Title: New parameter in 'content_structure_tree' operator.

*Documentation:
Added new boolean parameter in 'content_structure_tree' operator: $unfoldNodeID.
<code>
{set contentStructureTree = content_structure_tree( $rootNodeID, $classFilter,
                                                    $maxDepth, $maxNodes,
                                                    $sortBy, $fetchHidden,
                                                    $unfoldNodeID ) }
</code>

If $unfoldNodeID is not set or equal 0 (false) then 'content_structure_operator'
works as it worked before. Otherwise, $unfoldNodeID define a node which must be
unfolded. Note what node (defined by the $unfoldNodeID) can be only first-level
child of the root node (defined by the $rootNodeID). If node $unfoldNodeID is
not first-level child of the root node or cannot be fetched att all then
'content_structure_tree' operator works as it worked before too.