Fix grouping issue

When switching to a group that is not shown currently, the group headers
won't be updated. The root-cause was that the m_groups-cache must be reset
when the roles have been changed.

CCBUG: 289654

src/kitemviews/kfileitemmodel.cpp

@@ -367,6 +367,7 @@ void KFileItemModel::setRoles(const QSet<QByteArray>& roles)
         }
     }
 
+    m_groups.clear();
     resetRoles();
 
     QSetIterator<QByteArray> it(roles);