qdoc traverses index nodes when writing the help file and when writing the html docs

When qdoc reads an index file prior to generating the docs for a module, it marks each node it creates as an index node. These nodes are then not included when qdoc writes the index file for the module it is currently generating docs for. The HTML generator and the help file generator should also ignore these index nodes, but currently they are not ignoring them. In most cases this should cause no problems, but it is at least a performance issue.