Migration to gitlab

Migrate the project from GitHub.com to the RLP instance of GitLab (here).

I also use this opportunity to see what can get wrong in a brand new project installation. Fixed the following bugs:

• DllNotFoundException, in openvr_api.cs, because of a gitignore conflict (manifest.json and packages-lock were ignored when they shouldn't);
• OperatorGUI initialized in the wrong state, function returning to early because of a malformed catch.