qdoc: Standardize paths used in example and demo manifest files

There is no longer any difference in install directories between demos
and examples (QT_INSTALL_DEMOS is obsolete and points to the same dir
as QT_INSTALL_EXAMPLES for compatibility). This change removes any
special handling of demos' paths used in manifest xml, enabling the
demos to be visible in Creator welcome mode as well.

Change-Id: I5afdf578a42d001fab05979d3ea1102fd52b51da
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>