Model hierarchy within subject hierarchy

[cpinter]

In order to label the existing atlases in Slicer properly, we need a way to assign standard terminologies to models in a hierarchy. Instead of fixing the current model hierarchy (which is quite buggy and that's part of the reason SH is not based on the original MRML scene model), we can achieve this within subject hierarchy:

• Add a new column for color
  • Proposed design: new function in abstract plugin which SH plugins can implement if makes sense to show one color (models, segments, fiducials, ?)
  • Add option to SH tree view to whitelist/blacklist plugins. This will be done with two QLists that, if not empty, will specify which plugins are used for populating the context menu
  • On double-click on color, the terminology selector will pop up, which sets the color and writes the terminology in the proper place (the relevant SH plugins would do this).
  • Add a remove button to terminology selector (in segments it's mandatory but in other nodes it's not)
• Add opacity SH plugin that has a slider in a sub-menu of the action (similarly to slice spacing in slice view controller)
• Add color information to folder items, allow them to override branch color

[cpinter]

@lassoan I finished all items in this ticket. I think we are now ready to replace the model hierarchy to subject hierarchy in the Models module.

[cpinter]

Pull request for replacing model hierarchy with subject hierarchy in the Models module:
Slicer/Slicer#1054

This PR enables managing atlases in Slicer accoridng to the needs defined by the OpenAnatomy project. Expected integration is after the release of Slicer 4.10.1.