Add armature overall envelope radius

The overall envelope radius changes the ratio of all the envelopes of the
armature.
For a given envelope, its radius is given by:
radius = boneLength * boneRadiusRatio * overallRadiusRatio

See Issue #13680
Add independent opacity for the envelope

The opacity of the envelope is now set indepently from the opacity of
the bone, through the armature node.
In an effort to make things easier, the opacity of the envelopes is always
lesser or equal to the opacity of the bones (at least when interacting
with the GUI).
The implementation followed the specifity of the bone's opacity and color.

Finally, the widget representation needed to be tweak to be able to handle
geometry that are both transculent and opaque.

See Issue #13680
Adding front face culling to the envelopes

Also rectifying bug that made opaque and translucent render for both
when the bone is opaque and the envelope translucent.

See Issue #13680
Merge branch '13680-EnvelopesDisplay' into armatures

* 13680-EnvelopesDisplay:
  Adding front face culling to the envelopes
  Add independent opacity for the envelope
  Add armature overall envelope radius