Restructure the entire editor codebase to consistently match the message hierarchy

Closes #744

editor/src/messages/broadcast/broadcast_event.rs

@@ -0,0 +1,11 @@
+use crate::messages::prelude::*;
+
+use serde::{Deserialize, Serialize};
+
+#[derive(PartialEq, Eq, Clone, Debug, Serialize, Deserialize, Hash)]
+#[impl_message(Message, BroadcastMessage, TriggerEvent)]
+pub enum BroadcastEvent {
+	DocumentIsDirty,
+	ToolAbort,
+	SelectionChanged,
+}

editor/src/messages/broadcast/broadcast_message.rs

@@ -0,0 +1,23 @@
+use crate::messages::prelude::*;
+
+use serde::{Deserialize, Serialize};
+
+#[remain::sorted]
+#[impl_message(Message, Broadcast)]
+#[derive(PartialEq, Clone, Debug, Serialize, Deserialize)]
+pub enum BroadcastMessage {
+	// Sub-messages
+	#[remain::unsorted]
+	#[child]
+	TriggerEvent(BroadcastEvent),
+
+	// Messages
+	SubscribeEvent {
+		on: BroadcastEvent,
+		send: Box<Message>,
+	},
+	UnsubscribeEvent {
+		on: BroadcastEvent,
+		message: Box<Message>,
+	},
+}

editor/src/messages/broadcast/broadcast_message_handler.rs

@@ -0,0 +1,32 @@
+use crate::messages::prelude::*;
+
+use std::collections::HashMap;
+
+#[derive(Debug, Clone, Default)]
+pub struct BroadcastMessageHandler {
+	listeners: HashMap<BroadcastEvent, Vec<Message>>,
+}
+
+impl MessageHandler<BroadcastMessage, ()> for BroadcastMessageHandler {
+	#[remain::check]
+	fn process_message(&mut self, message: BroadcastMessage, _data: (), responses: &mut VecDeque<Message>) {
+		#[remain::sorted]
+		match message {
+			// Sub-messages
+			#[remain::unsorted]
+			BroadcastMessage::TriggerEvent(event) => {
+				for message in self.listeners.entry(event).or_default() {
+					responses.push_front(message.clone())
+				}
+			}
+
+			// Messages
+			BroadcastMessage::SubscribeEvent { on, send } => self.listeners.entry(on).or_default().push(*send),
+			BroadcastMessage::UnsubscribeEvent { on, message } => self.listeners.entry(on).or_default().retain(|msg| *msg != *message),
+		}
+	}
+
+	fn actions(&self) -> ActionList {
+		vec![]
+	}
+}

editor/src/messages/broadcast/mod.rs

@@ -0,0 +1,9 @@
+mod broadcast_message;
+mod broadcast_message_handler;
+
+pub mod broadcast_event;
+
+#[doc(inline)]
+pub use broadcast_message::{BroadcastMessage, BroadcastMessageDiscriminant};
+#[doc(inline)]
+pub use broadcast_message_handler::BroadcastMessageHandler;