Adopt EventEmitter class instead of injecting Signal methods

Introduce a new class, EventEmitter, which implements signal
handling for pure JavaScript classes. EventEmitter still
utilizes GJS' addSignalMethods internally.

EventEmitter allows static typechecking to understand the
structure of event-emitting JS classes and makes creating
child classes simpler.

The name 'EventEmitter' mirrors a common name for this pattern
in Node and in JS libraries.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2043>

js/gdm/credentialManager.js

@@ -1,8 +1,12 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
 /* exported CredentialManager */
 
-var CredentialManager = class CredentialManager {
+const Signals = imports.misc.signals;
+
+var CredentialManager = class CredentialManager extends Signals.EventEmitter {
     constructor(service) {
+        super();
+
         this._token = null;
         this._service = service;
         this._authenticatedSignalId = null;