Doc: Setting \qtvariable in module page.

\qtvariable indicates the QT variable needed in .pro files.
QDoc will place the QT variable in the class documentation.

Change-Id: I22437154f6808f9eba810d0e35ccae59782fc051
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Martin Smith <martin.smith@digia.com>