Group by the header the user has clicked on.
@private
function(menuItem, e) {
        var me = this,
            menu = menuItem.parentMenu,
            hdr  = menu.activeHeader,
            view = me.view,
            store = view.store;

        delete me.lastGroupIndex;
        me.block();
        me.enable();
        store.group(hdr.dataIndex);
        me.pruneGroupedHeader();
        me.unblock();
        me.refreshIf();
    }