[dvdplayer] calculate seekoffset when performing chapter seeks

This calculates the missing seekoffset value when performing chapter seeks and pushes it the infomanager. Time/percentage bases seeks already do this (kind of). I am not entirely sure if it's done right. There might be a better way. @FernetMenta for review please.

Bug report @ http://forum.kodi.tv/showthread.php?tid=210778

would be easier to follow and more robust if you used start instead of GetTime here. why do you device by 1000? all values should be milliseconds.