[Device] Add GotoTrack() player method

- Add a new Player interface method GotoTrack(...) which will
perform a seek operation with "TRACK_NR" unit on the specified
track number.
- Add CurrentTrack and NumberOfTracks properties to the Player
interface.
- API documentation updated
- Tested with Rygel playlist

Fix isuue: https://github.com/01org/renderer-service-upnp/issues/90

Signed-off-by: Christophe Guiraud <christophe.guiraud@intel.com>