Fix keyword index generation for qch

Previously due this bug all C++ context searches were pointing to "cpp" index
page, C searched to "c" index page. This fix brings back context search (F1)
functionality that was broken for last 2014 qch releases.

This bug was introduced by 9b020fd7e1983c584b72483edd2968720b477d25 rewrite of
index generation from XSL to Python.