CQ5 - disable dialog field when content is loaded

## dialog.xml
<?xml version="1.0" encoding="UTF-8"?>
<jcr:root xmlns:cq="http://www.day.com/jcr/cq/1.0" xmlns:jcr="http://www.jcp.org/jcr/1.0" jcr:primaryType="cq:Dialog"
    xtype="dialog">
    <listeners jcr:primaryType="nt:unstructured"
        loadcontent="function(dialog) {
            // call servlet or perform arbitrary javascript functions

            dialog.getField('./operator').disable();
        }" />
    <items jcr:primaryType="cq:WidgetCollection">
        <tabs jcr:primaryType="cq:TabPanel">
            <items jcr:primaryType="cq:WidgetCollection">
                <tab jcr:primaryType="cq:Widget" title="Profile Property" xtype="panel">
                    <items jcr:primaryType="cq:WidgetCollection">
                        <operator jcr:primaryType="cq:Widget" fieldLabel="Property Name" name="./operator"
                            type="select" xtype="selection" optionsProvider="getPropertyNameOptions"
                            allowBlank="{Boolean}false" />
                    </items>
                </tab>
            </items>
        </tabs>
    </items>
</jcr:root>