Add vtkCapsuleSource

vtkCapsuleSource is polydata algorithm that generates a polygonal capsule.

See Issue #13680
Add Envelope representation

Very simple early stage, just adding and envelope representation to
all the bone representation.

See Issue #13680
Make envelopes available in Slicer

It was decided for this property to be part of the bone display node.
It is updated on the armature and then updated on each display node from
the armature node.
Note that only the armature node should be needed once 1779 is implemented.

See Issue #13680
Rename EnvelopeVisible to ShowEnvelope

This will make the naming more consistent.

See Issue #13680
Add envelope radius slider

Adding a slider to control the envelope radius. Also adding the forgotten
tooltips at the same time. Much like the show envelope checkbox, this
could be simplified with issue 13779.

Note: The maximum radius was set to 100 but it is very much arbitrary.

See Issue #13680
Add radius field data to the armature polydata

See Issue #13680