@Override
    protected void onLoad() {
        DOM.setEventListener(inputElem, this);
    }
This method is called when a widget is attached to the browser's document.
onAttach needs special handling for the CheckBox case. Must still call
{@link Widget#onAttach()} to preserve the <code>onAttach</code> contract.
stroom-core-client-widget/src/main/java/stroom/widget/tickbox/client/view/CustomCheckBox.java