Draw (WORKPIECE,...) stock as a translucent solid

[alex-pres]

See #4429

New glcanon.workpiece_opacity option, a float 0..1 defaulting to 0 (outline only), drawing lit faces under the existing wireframe. Every host exposes it.

Added menu option for axis, qt5_graphics also have the option but that is not exposed to UI.

[andypugh]

I like it, but then I suggested it at the Stuttgart meeting. (I am not sure if this is just coincidence)

[hansu]

I like it too!
Only the variable names seems a bit long - maybe they can be shortened to DIA, INNER_DIA, or even D_O, D_I or something like that.

[andypugh]

I like it too! Only the variable names seems a bit long - maybe they can be shortened to DIA, INNER_DIA, or even D_O, D_I or something like that.

"ID" and "OD" are in general use in (for example) technical drawings.