qdoc should report duplicate page titles better

Currently, qdoc reports duplicate pages, when it should be reporting duplicate page titles. Sometimes the duplicate titles actually refer to the same page, but often they are different pages with the same title.

A further complication is that the qdoc warnings for these duplate page title errors are useless when the duplicates are in different Qt5 modules, because the support for file location information in the qdoc index files is inadequate.