Doc: Fixed section in "All Modules" page about the QT variable.

-Fixes missing snippet issue.
-removed snippet file because the snippet is only one line.
-put the snippet content in \code \endcode.
-linked to Qt Core and Qt GUI pages.

Change-Id: If91d071fd572e39fdd532054cdc1626eb915595e
Reviewed-by: Martin Smith <martin.smith@digia.com>