Restructure the entire editor codebase to consistently match the message hierarchy

Closes #744

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![]
+	}
+}