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/ui/status/thunderbolt.js

@@ -4,7 +4,7 @@
 // the following is a modified version of bolt/contrib/js/client.js
 
 const { Gio, GLib, GObject, Polkit, Shell } = imports.gi;
-const Signals = imports.signals;
+const Signals = imports.misc.signals;
 
 const Main = imports.ui.main;
 const MessageTray = imports.ui.messageTray;
@@ -49,8 +49,10 @@ const BOLT_DBUS_CLIENT_IFACE = 'org.freedesktop.bolt1.Manager';
 const BOLT_DBUS_NAME = 'org.freedesktop.bolt';
 const BOLT_DBUS_PATH = '/org/freedesktop/bolt';
 
-var Client = class {
+var Client = class extends Signals.EventEmitter {
     constructor() {
+        super();
+
         this._proxy = null;
         this.probing = false;
         this._getProxy();
@@ -127,11 +129,12 @@ var Client = class {
         return this._proxy.AuthMode;
     }
 };
-Signals.addSignalMethods(Client.prototype);
 
 /* helper class to automatically authorize new devices */
-var AuthRobot = class {
+var AuthRobot = class extends Signals.EventEmitter {
     constructor(client) {
+        super();
+
         this._client = client;
 
         this._devicesToEnroll = [];
@@ -217,7 +220,6 @@ var AuthRobot = class {
         return GLib.SOURCE_REMOVE;
     }
 };
-Signals.addSignalMethods(AuthRobot.prototype);
 
 /* eof client.js  */