GitLab: Introduce minimal GitLab plugin

So far only adding global settings and project settings.
Global settings allow to specify configurations used later
inside the project settings or when accessing GitLab
server instances.
Project settings already can "link" to GitLab projects,
which will allow to fetch notifications for the project
later on.
Real functionality is added in the follow ups.

Change-Id: I8a0f978001c58c6dc9e07917c183891abec9a3d0
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/plugins.qbs

@@ -40,6 +40,7 @@ Project {
         "emacskeys/emacskeys.qbs",
         "genericprojectmanager/genericprojectmanager.qbs",
         "git/git.qbs",
+        "gitlab/gitlab.qbs",
         "glsleditor/glsleditor.qbs",
         "helloworld/helloworld.qbs",
         "help/help.qbs",