Proxies in nested ProxyProperty are missing from XML State

Proxies, that are set by a ProxyProperty and have a ProxyProperty themselves are missing their proxies in the XML state.

For example, the following MyStreamTracer proxy has a SplineSource attached to a InputProperty, which has a Spline proxy attached to a ProxyProperty.

<SourceProxy name="MyStreamTracer"
             class="vtkStreamTracer">
    <InputProperty command="SetSourceConnection" name="Source">
        <ProxyListDomain name="proxy_list">
            <Proxy group="sources" name="SplineSource" />
        </ProxyListDomain>
    </InputProperty>

...

<SourceProxy class="vtkParametricFunctionSource"
             name="SplineSource">
  <ProxyProperty command="SetParametricFunction" name="ParametricFunction">
    <ProxyListDomain name="proxy_list">
      <Proxy group="parametric_functions" name="Spline" />
    </ProxyListDomain>
  </ProxyProperty>

Saving a state containing a MyStreamTracer will correctly save the SplineSource proxy:

    <Proxy group="sources" type="SplineSource" id="17080" servers="1">
      <Property name="ParametricFunction" id="17080.ParametricFunction" number_of_elements="1">
        <Proxy value="17079"/>

but the proxy with ID 17079 is missing from the XML state. This could be related to it not being registered as helper proxy of MyStreamTracer1.

Steps to reproduce

1. Load plugin.xml
2. Create Wavelet source, attach MyStreamTracer filter
3. Save state (produces state.pvsm)
4. Load state

The loaded MyStreamTracer1 has now an empty Parametric Function property.