Lab: Add an operation to create triangle faces from vertex selections

[MaelRL]

Summary of Changes

• Create a selection item
• Select (exactly) three vertices
• Choose "Add triangle face from selected vertices"

Release Management

• Affected package(s): lab
• Issue(s) solved (if any): -
• Feature/Small Feature (if any): -
• License and copyright ownership: no change

[sloriot]

Successfully tested in CGAL-6.0-Ic-274