@Override
            public void onClick(ClickEvent event) {
                // Checkboxes always toggle their value, no need to compare
                // with old value. Radio buttons are not so lucky, see
                // overrides in RadioButton
                ValueChangeEvent.fire(CustomCheckBox.this, getValue());
            }
Checks or unchecks the check box, firing {@link ValueChangeEvent} if
appropriate.
<p>
Note that this <em>does not</em> set the value property of the checkbox
input element wrapped by this widget. For access to that property, see
{@link #setFormValue(String)}

@param value      true to check, false to uncheck; null value implies false
@param fireEvents If true, and value has changed, fire a
                  {@link ValueChangeEvent}
stroom-core-client-widget/src/main/java/stroom/widget/tickbox/client/view/CustomCheckBox.java