Rygel 0.21.2 Media Server Adds More Samsung Hacks

When compared with the previous development release (0.21.1), we can notice that Rygel 0.21.2 adds support for Tracker 0.18, adds more Samsung hacks, introduces seeking hacks for various UPnP music receivers, and allows server plugins to override the search capabilities. In addition, Rygel 0.21.2 implements SetNextAVTransportURI, treats REL_TIME/ABS_TIME like before, uses the correct PlaybackStorageMedium values, removes unused state variable, sets default play speed to prevent a critical MPRIS error, and fixes the wrong parameter order in AlbumArtFactory.