Add GitLab issue tracker support

[Maxhy]

GitLab issue tracker is currently not supported and could be easily integrated once https://gitlab.com/gitlab-org/gitlab-ce/issues/30822 implemented. For now, it may be possible to do manually the filter on client side based on closed_at (/api/v4/projects/myProject/issues?state=closed). Issue for tracking & discussion before implementation.

[Maxhy]

I did a first implementation but I'm not able to test it very well... Looks like I also have to update GitTools.IssueTrackers.IssueTrackerType to add GitLab on the list. What I did locally but my new factory is still not detected on GitReleaseNotes. What's the magic here? Could you explain quickly here so I will send a PR for this support? Thanks.