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 16 seconds (queued for 53 seconds)
latest merge request
Name Stage Failure
failed
run-checks Checks
Entering 'libvlcpp'
Entering 'test/samples/samples'
Entering 'libvlcpp'
Entering 'test/samples/samples'
Executing "step_script" stage of the job script
Using docker image sha256:7e547830a7015f5c0239a494153b268d6a86865e8a1fbd391b519bace11a0de4 for registry.videolan.org/medialibrary:20211029121143 with digest registry.videolan.org/medialibrary@sha256:464a2d8044ef7489a9757d0015aa29284347cb23dfccb72af3e04ca5b07a4a00 ...
$ if [ -z "$CI_MERGE_REQUEST_MILESTONE" ]; then echo "A milestone is required for new merge requests"; exit 1; fi
A milestone is required for new merge requests
Cleaning up file based variables
ERROR: Job failed: exit code 1