lib/lp-media.c: Add rate parameter to lp_media_seek.

[rodrimc]

Preliminary tests with positive rates (fast-forwarding) are promising. But in order for it to work we need to add the pitch filter (or any other filter that handles rate changes) to the audio graph of media objects. Negative rates are more problematic though. They didn't work as expected probably due to the overhead imposed on the decoder. Maybe we could use buffer queues (or uridecodebin buffering) to alleviate the decoder's overhead.