rem: KMHeaderItem( QListView* parent, int msgId, QString key = QString::null)
add: KMHeaderItem( QListViewItem* parent, int msgId, QString key = QString::null)
context:     KMHeaderItem( QListView* parent, int msgId, QString key = QString::null)    : KListViewItem( parent ),          mMsgId( msgId ),          mKey( key ),          mAboutToBeDeleted( false ),          mSortCacheItem( 0 )  {    irefresh();  }