Restructure the entire editor codebase to consistently match the message hierarchy

Closes #744

editor/src/messages/mod.rs

@@ -0,0 +1,14 @@
+//! The root-level messages forming the first layer of the message system architecture.
+
+pub mod broadcast;
+pub mod debug;
+pub mod dialog;
+pub mod frontend;
+pub mod input_mapper;
+pub mod input_preprocessor;
+pub mod layout;
+pub mod message;
+pub mod portfolio;
+pub mod prelude;
+pub mod tool;
+pub mod workspace;