Service Catalog tree fixed to not show Actions nodes below Service nodes

Moved x_get_tree_st_kids method from parent TreeBuilderCatalogsClass to subclasses TreeBuilderCatalogItems and TreeBuilderServiceCatalog, so each can determine what to draw in the tree.
Merge pull request #164 from dclarizio/fix_service_tree

Service Catalog tree fixed to not show Actions nodes below Service nodes