Hierarchy visibility refactoring

[cpinter]

The are two main goals of this PR:

• Change the way visibility works in subject hierarchy for folders
• Remove model hierarchies from Slicer

This is the related issue: PerkLab/SlicerOpenAnatomy#4

Details:

1. Use only folders in subject hierarchy instead of the former hybrid folder / model hierarchy way (which had some issues, such as the inability to create folders that allow overriding display properties, only programmatically or opening a scene).
   • Convert model hierarchies to SH folders on scene import
   • Change SH node visibility functions to only change visibility in the directly or indirectly associated display node, instead of applying the visibility setting on all children individually
   • Folder items have a directly associated display node after any interaction with display settings of the folder. A new display node type vtkMRMLFolderDisplayNode is added for this
   • DICOM type hierarchy items (subject, study) now subclass folders, inheriting all its features
   • Displayable managers now consider only folders, not model hierarchies. If a folder has apply display properties on branch enabled, then the model uses the display node of the folder for all properties
     • Visibility of the parent items is always considered (i.e. if a parent is hidden, the branch is hidden, regardless the override settings)
     • Opacity of the parent items is always considered (i.e. multiply the opacity values of the branch)
     • Displayable managers that now support overrides: Model 3D+2D, Segmentations 3D+2D, Markups (for markups it's not the DM but the VTKWidget that applies the display properties)
   • Add new event SubjectHierarchyItemReparentedEvent to SH. It is used to update display if models after reparenting (may be dropped under a folder that overrides display properties)
   • Display nodes have a FolderDisplayOverrideAllowed flag, which determines whether folder nodes can override their display properties. On by default
   • Handle multi-select in models SH (change opacity, set slice intersection etc for multiple models)
2. Remove model hierarchy
   • Remove ModelHierarchy mode from qMRMLTreeView
   • Remove ModelHierarchyDisplayNodeClassName concept from selection node
   • Update segmentation import/export from/to model hierarchies to use folders instead where appropriate
     • Use subject hierarchy combobox instead of MRML node combobox in Import/export section in Segmentations module
     • Add option to filter for multiple levels in subject hierarchy
   • Update qMRMLModelDisplayNodeWidget to support folders
     • Set SH item instead of "ModelOrHierarchyNode"
     • Use generic display node and only show model display node features if item of model node is set
   • Remove fiber related features from Models module UI (@pieper will later re-add SH features for this with my help)

Questions:

• ScenePerformance.py - What are the model hierarchy tests measuring? Should I add folder display nodes instead?

Post-PR tasks (while we discuss the PR and while I fix any occasional things to fix):

• "Convert model hierarchy to segmentation node" also available from Folders SH plugin
• Cache overriding folder node reference in display nodes (if getting overriding folder turns out to be a bottleneck)

[cpinter]

If there are no comments, I will integrate this tonight.

@jcfr Can you please answer the question I had with one of the tests? It seems you added this test. Thanks!

ScenePerformance.py - What are the model hierarchy tests measuring? Should I add folder display nodes instead?

[cpinter]

Integrated in
Slicer/Slicer@3813e8d
Slicer/Slicer@637cf7c
Slicer/Slicer@747cab1
Slicer/Slicer@c5fe138
Slicer/Slicer@b849223
Slicer/Slicer@2564269
Slicer/Slicer@4f5c2f4
Slicer/Slicer@4504085
Slicer/Slicer@659f1c0

[cpinter]

If there is any issue, I'll be happy to fix it.

@jcfr please answer the (btw minor) question about ScenePerformance.py, just so that it's not forgotten.