Sets the read only state of this field.
@param {Boolean} readOnly Whether the field should be read only.
function(readOnly) {
        var me = this,
            inputEl = me.inputEl;
        readOnly = !!readOnly;
        me[readOnly ? 'addCls' : 'removeCls'](me.readOnlyCls);
        me.readOnly = readOnly;
        if (inputEl) {
            inputEl.dom.readOnly = readOnly;
        } else if (me.rendering) {
            me.setReadOnlyOnBoxReady = true;
        }
        me.fireEvent('writeablechange', me, readOnly);
    }