Re: [Tracker] Add support for OMA DRM Content format

["Lin, Mengdong" <mengdong lin intel com>]

There may be a better way:
How about introducing a "priority" or "rank" property for the extractors? Once a extractor is loaded, it's 
order in the specific/general extractor list is decided by its priority. If more than one extractors match a 
mime type, the higher-priority one will be tired before the lower-priority one.

Then I can still write a single DCF extractor as a fallback for the GStreamer extractor:
*  Both DCF and GStreamer extractor support the DRM format "application/vnd.oma.drm.content":
*  GStreamer extractor has a higher priority; if DRM-aware GStreamer extension is installed, this extractor 
can get rich meta data from decodebin2 from the decrypted content; Otherwise, decodebin2 can neither 
recognize the file type not decrypt the content, thus nothing can be extracted.
*  DCF extractor has a lower priority, it can only get the original media MIME type from the unencrypted DCF 
file header. It is unable to decrypt the content. 
*  The tracker-extract will try GStreamer extractor at first. If GStreamer extractor gives thing, it will try 
DCF extractor.

This method can allow other general extractors to support OMA DRM formats.

Thanks & Best Regards
Amanda