Make documents searchable by words found in the text body of text and picture annotations

We would like to automatically index the text content of annotation as part of the default fulltext index of the annotated documents.

Implementation plan:

• introduce a new schema "related_indexable_text" with a list of complex properties ("related_id", "indexable_text")
• introduce a new dynamic facet "RelatedIndexableText" that manages the "related_indexable_text" schema
• introduce a new core event listener that watches the "AnnotationCreated" / "AnnotationDeleted" events, and attach the facet "RelatedIndexableText" to the annotated document and add or remove an entry in the related_indexable_text schema with "related_id" set to "annotation:${annotation_uid}" and the content of the text annotation in "indexable_text".

This "RelatedIndexableText" schema is meant to be generic enough to be able to index other related text content such as comments, relation literals and tags that are not stored as nuxeo fields.