Constrain fiducials (WIP, do not merge)
This works pretty well, and can be a placeholder until the more comprehensive markup overhaul is ready.
You might want to try it and see if we can figure out how to make the fiducial 'stick' to the mouse, but if not I think okay as-is and better than the default behavior. I'd be tempted to include it as-is in the 4.10.2 patch release.
Note that I looked into making it possible to toggle this behavior, but didn't see how to get access to state from a QSetting into a vtkMRMLDisplayableManager (some options discussed in the commit message). @jcfr or @lassoan any thoughts?