Added unified, simple event bus to eventually replace controller "notifiers"

src/main/java/org/qortal/event/Event.java

@@ -0,0 +1,5 @@
+package org.qortal.event;
+
+public interface Event {
+
+}

src/main/java/org/qortal/event/EventBus.java

@@ -0,0 +1,33 @@
+package org.qortal.event;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public enum EventBus {
+	INSTANCE;
+
+	private static final List<Listener> LISTENERS = new ArrayList<>();
+
+	public void addListener(Listener newListener) {
+		synchronized (LISTENERS) {
+			LISTENERS.add(newListener);
+		}
+	}
+
+	public void removeListener(Listener listener) {
+		synchronized (LISTENERS) {
+			LISTENERS.remove(listener);
+		}
+	}
+
+	public void notify(Event event) {
+		List<Listener> clonedListeners;
+
+		synchronized (LISTENERS) {
+			clonedListeners = new ArrayList<>(LISTENERS);
+		}
+
+		for (Listener listener : clonedListeners)
+			listener.listen(event);
+	}
+}

src/main/java/org/qortal/event/Listener.java

@@ -0,0 +1,6 @@
+package org.qortal.event;
+
+@FunctionalInterface
+public interface Listener {
+	void listen(Event event);
+}