paved the way for private SPU commands.

Add a PLAYLIST_PREPARSE flag to tell that the item must be enqueued for preparse on add (not implemented yet).

Refs:#192

dshow.cpp: Removed default preferred media types (I420) as it was reported by Paul Sokolovsky to cause kernel crashes
           with some buggy drivers. I added these media types a while back to get around another bug for drivers that
           didn't offer any preferred media types.
           Hopefully, the latter bug should be indirectly fixed when I added support for IAMStreamConfig.
           Moreover, I modified the selection of a default format in IAMStreamConfig to take
           GetFourCCPriority into a account.