Make it easier to access tags, in the Contact and Organizer APIs.

Probably something like:

QSet<QString> QContact::tags() const;
void QContact::setTags(const QSet<QString>& tags);

// also probably worthwhile
void QContact::addTag(const QString& tag);
void QContact::removeTag(const QString& tag);

// also possible
bool QContact::hasTag(const QString& tag) const {return tags().contains(tag);}