Toggle the ui header between checked and unchecked state.
@param {Boolean} isChecked
@private
function(isChecked) {
        var view     = this.views[0],
            headerCt = view.headerCt,
            checkHd  = headerCt.child('gridcolumn[isCheckerHd]'),
            cls = this.checkerOnCls;

        if (checkHd) {
            if (isChecked) {
                checkHd.addCls(cls);
            } else {
                checkHd.removeCls(cls);
            }
        }
    }