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/misc/objectManager.js

@@ -1,8 +1,9 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
+/* exported ObjectManager */
 
 const { Gio, GLib } = imports.gi;
 const Params = imports.misc.params;
-const Signals = imports.signals;
+const Signals = imports.misc.signals;
 
 // Specified in the D-Bus specification here:
 // http://dbus.freedesktop.org/doc/dbus-specification.html#standard-interfaces-objectmanager
@@ -25,8 +26,10 @@ const ObjectManagerIface = `
 
 const ObjectManagerInfo = Gio.DBusInterfaceInfo.new_for_xml(ObjectManagerIface);
 
-var ObjectManager = class {
+var ObjectManager = class extends Signals.EventEmitter {
     constructor(params) {
+        super();
+
         params = Params.parse(params, {
             connection: null,
             name: null,
@@ -288,4 +291,3 @@ var ObjectManager = class {
         return proxies;
     }
 };
-Signals.addSignalMethods(ObjectManager.prototype);