@Override
    public int getVisibleItemCount() {
        return presenter.getVisibleItemCount();
    }
Get the key for the specified value. If a keyProvider is not specified or the value is null,
the value is returned. If the key provider is specified, it is used to get the key from
the value.

@param value the value
@return the key
stroom-core-client-widget/src/main/java/com/google/gwt/user/cellview/client/AbstractHasData.java