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