qdoc: Restore support for output in one directory

The basic functionality is working. Add these lines
to qt-html-templates-offline.qdocconf, or add them
to the online version:

HTML.nosubdirs            = "true"
HTML.outputsubdir         = "html"

Before it opens a .html file for writing, it tests
whether the file alread exists. If so, it writes an
error message, e.g.:

...platform-notes.qdoc:140: error: HTML file already exists;
overwriting .../doc/html/platform-notes-windows.html

There are currently nearly 100 files being overwritten for Qt5.

Task-number: QTBUG-32580
Change-Id: I02b103fd00b9d1e624665ac518d571acc791be9d
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>