Add armature model with slicer IO system

Instead of returning a collection we can simply return an armature. This
makes things easier and one can still access the bones through the armature
node.
Since we are now using slicer's IO, deleting the "Load from Model" button.

See Issue #13965
Merge branch '13965-ArmatureFromVTKFile' into armatures

* 13965-ArmatureFromVTKFile:
  Add armature model with slicer IO system