Support the once option, when registering EventBus listeners

This follows the same principle as the `once` option that exists in the native `addEventListener` method, and will thus automatically remove an `EventBus` listener when it's invoked; see https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#Parameters

Finally, this patch also tweaks some the existing `EventBus`-code to use modern features such as optional chaining and logical assignment operators.

test/unit/ui_utils_spec.js

@@ -312,6 +312,30 @@ describe("ui_utils", function () {
       expect(count).toEqual(2);
     });
 
+    it("dispatch event to handlers with/without 'once' option", function () {
+      const eventBus = new EventBus();
+      let multipleCount = 0,
+        onceCount = 0;
+
+      eventBus.on("test", function () {
+        multipleCount++;
+      });
+      eventBus.on(
+        "test",
+        function () {
+          onceCount++;
+        },
+        { once: true }
+      );
+
+      eventBus.dispatch("test");
+      eventBus.dispatch("test");
+      eventBus.dispatch("test");
+
+      expect(multipleCount).toEqual(3);
+      expect(onceCount).toEqual(1);
+    });
+
     it("should not re-dispatch to DOM", function (done) {
       if (isNodeJS) {
         pending("Document in not supported in Node.js.");