Fix document preview when the main blob is modified after annotation

enable annotations with the following contribution:

<require>org.nuxeo.ecm.platform.annot.properties</require>
<extension target="org.nuxeo.runtime.ConfigurationService" point="configuration">
    <property name="nuxeo.text.annotations">true</property>
</extension>

1) create a File document
2) attach a PDF
3) add an annotation
4) modifiy the blob by uploading another PDF for example
5) version number increment
6) go to More then Preview
7) the first PDF is displayed with the annotions