feat: user-defined physical materials stored in the design (#532) (#606)

A design can define its own physical materials (material.create, material.edit, material.delete): name, density and an optional look. They work everywhere built-in materials do: assignment, mass properties, colours and configuration cells. Stored in user_materials, an additive field under format 16. Un-ignores the #554 regression test. Fixes #532.

Co-authored-by: jonaswre <jonas.wrede@beyondit.gmbh>

crates/doc/src/format.rs

@@ -60,6 +60,9 @@
 //!   `font` is in the built-in font, as it always was.
 //!   A Move feature can also keep its source bodies and transform a copy (`copy: true`); a
 //!   move without it saves as before.
+//!   A design can also define its own physical materials (`user_materials`: name, density in
+//!   g/cm³, optional look), which bodies (`materials`) and configuration material cells name
+//!   like built-in ones; a design without them saves as before.
 //!   A design can also keep what was hidden when it was saved (`visibility`: hidden bodies,
 //!   occurrence placements, sketches and origin items; `browser.visibility`). A design without
 //!   it shows everything, as it always did; a reader that ignores it loses only the hiding.