freecad: makeThickSolid shell helper

[St0nefish]

Problem

There is no helper to shell a solid to a uniform wall thickness (wrapping OCC's
makeThickSolid). Without it, dust hoods / printed parts that want sloped or
funneled outer shapes have to be built with a hand-rolled box-minus-cavity
approach, which forces a flat top.

Migrated from freecad-plugin-gaps.md (nice-to-haves), quoted in full:

A helper to shell a solid with uniform wall thickness (OCC makeThickSolid)
would let dust hoods / printed parts have sloped or funneled outer shapes
without the hand-rolled box-minus-cavity approach (which forces a flat top).

Source

Migrated from freecad-plugin-gaps.md in the freecad-woodworking project
(nice-to-haves section) — also called out in that file's "still open" recap as
"makeThickSolid shell helper".