Local appearance and material libraries (#863)

Adds local scalar appearance and material libraries: a catalog in the io crate with safe writes, engine commands to list and apply library entries, and a library dialog in the UI, with docs.

crates/engine/src/cmd/mod.rs

@@ -38,6 +38,7 @@ mod config;
 mod file;
 mod inspect;
 pub(crate) mod joints;
+mod libraries;
 mod material_properties;
 mod materials;
 mod measure_sel;
@@ -254,6 +255,7 @@ pub fn command_specs() -> Vec<&'static CommandSpec> {
     v.extend(inspect::COMMANDS.iter());
     v.extend(materials::COMMANDS.iter());
     v.extend(material_properties::COMMANDS.iter());
+    v.extend(libraries::COMMANDS.iter());
     v.extend(section::COMMANDS.iter());
     v.extend(view_navigation::COMMANDS.iter());
     v.extend(diagnostics::COMMANDS.iter());