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/magnifier.js

@@ -1,9 +1,10 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
+/* exported Magnifier */
 
 const {
     Atspi, Clutter, GDesktopEnums, Gio, GLib, GObject, Meta, Shell, St,
 } = imports.gi;
-const Signals = imports.signals;
+const Signals = imports.misc.signals;
 
 const Background = imports.ui.background;
 const FocusCaretTracker = imports.ui.focusCaretTracker;
@@ -88,8 +89,10 @@ var MouseSpriteContent = GObject.registerClass({
     }
 });
 
-var Magnifier = class Magnifier {
+var Magnifier = class Magnifier extends Signals.EventEmitter {
     constructor() {
+        super();
+
         // Magnifier is a manager of ZoomRegions.
         this._zoomRegions = [];
 
@@ -724,7 +727,6 @@ var Magnifier = class Magnifier {
         }
     }
 };
-Signals.addSignalMethods(Magnifier.prototype);
 
 var ZoomRegion = class ZoomRegion {
     constructor(magnifier, mouseSourceActor) {