Fix picture views generation with auto versioning

With auto versioning enabled on documents with the Picture facet, the whole picture views generation process is triggered when creating the new version, even if the picture views are already computed on the document.

This is due to the auto versioning triggering a aboutToCheckIn event not caught by the PictureChangedListener, so the param disabling the generation cannot be set.

In the PictureChangedListener, we also need to check the aboutToCheckIn event and allow the picture views generation only if there are not already computed.

Auto versioning contribution for testing:

<extension target="org.nuxeo.ecm.core.api.versioning.VersioningService" point="policies">
  <policy id="picture-policy" order="1" beforeUpdate="true" increment="MAJOR">
    <filter-id>picture-filter</filter-id>
  </policy>
</extension>

<extension target="org.nuxeo.ecm.core.api.versioning.VersioningService" point="filters">
  <filter id="picture-filter">
    <facet>Picture</facet>
  </filter>
</extension>