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.

docs/book/src/SUMMARY.md

@@ -22,6 +22,7 @@
 - [Contributing](dev/contributing.md)
 - [Architecture](dev/architecture.md)
 - [Extensible library proposal](dev/libraries.md)
+  - [Local scalar libraries](dev/local-libraries.md)
 - [The command engine](dev/commands.md)
 - [The kernel boundary and truck](dev/kernel.md)
 - [Persistent naming](dev/naming.md)