Fix resume playback from playlist does not work (trac 13929)

It appears that the root cause is that playlist GUI uses copies of the FileItems that are in the library. The result of this that the resumepoint and startoffset are set the wrong place (the "gui" item) and then the playlistplayer starts the playback of the actual playlist item (which is the same as the library one). 

Next to the problem described in the ticket (cannot resume in playlist) this has the additional effect that when playback of a movie via the library, the resume point in the playlist GUI "playing now..." is out of date, because it does not get refreshed.