Add some internal API for extracting a QSharedPointer<T> from QVariant.

Add some internal API for extracting a QSharedPointer<T> from QVariant.

The T must be derived from QObject, or it will fail to compile.

This will allow scripting or other 'wrapping' and runtime environments like
QtDeclarative to handle QSharedPointers to types derived from QObject
properly. A QSharedPointer<T> can be inserted into a QVariant, and
where T derives from QObject, a QSharedPointer<QObject> can be
extracted from the QVariant, and its properties are then accessible.

Change-Id: I68d6d89aceceb019267bd7301baa2047f9c09b90
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>