Restructure the entire editor codebase to consistently match the message hierarchy

Closes #744

editor/src/messages/dialog/mod.rs

@@ -0,0 +1,18 @@
+//! Handles modal dialogs that appear as floating menus in the center of the editor window.
+//!
+//! Dialogs are represented as structs that implement the `PropertyHolder` trait.
+//!
+//! To open a dialog, call the function `register_properties` on the dialog struct with `responses` and the `LayoutTarget::DialogDetails` enum variant.
+//! Then dialog can be opened by sending the `FrontendMessage::DisplayDialog` message;
+
+mod dialog_message;
+mod dialog_message_handler;
+
+pub mod export_dialog;
+pub mod new_document_dialog;
+pub mod simple_dialogs;
+
+#[doc(inline)]
+pub use dialog_message::{DialogMessage, DialogMessageDiscriminant};
+#[doc(inline)]
+pub use dialog_message_handler::DialogMessageHandler;