This adds extractor metadata in a backwards and forward compatible way.
There are several use cases for this metadata:
- Delayed loading of extractor plugins - currently, all extractors are
loaded and and initialized when an ExtractorCollection is created.
- Versioning information - e.g. Baloo would benefit from versioning
information, to reindex affected files after an extractor has been
updated.
Although it would be possible to extend the extractor plugin interface
with a method for each relevant property, it would require a bump of
the plugin inteface version each time the interface is extended.