Restructure the entire editor codebase to consistently match the message hierarchy

Closes #744

editor/src/messages/layout/layout_message.rs

@@ -0,0 +1,14 @@
+use super::utility_types::misc::LayoutTarget;
+use crate::messages::layout::utility_types::layout_widget::Layout;
+use crate::messages::prelude::*;
+
+use serde::{Deserialize, Serialize};
+
+#[remain::sorted]
+#[impl_message(Message, Layout)]
+#[derive(PartialEq, Clone, Debug, Serialize, Deserialize)]
+pub enum LayoutMessage {
+	RefreshLayout { layout_target: LayoutTarget },
+	SendLayout { layout: Layout, layout_target: LayoutTarget },
+	UpdateLayout { layout_target: LayoutTarget, widget_id: u64, value: serde_json::Value },
+}