Media: add isUnseen accessor

It will return true if the playCount was set to 0, and if lastPosition
is negative which means the media was never played or that it ended
(in that case, the playCount won't be equal to 0)

Ref. #388
10 jobs for !534 with issue-388 in 5 minutes and 42 seconds (queued for 40 seconds)
latest merge request