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/ibusManager.js

@@ -2,7 +2,7 @@
 /* exported getIBusManager */
 
 const { Gio, GLib, IBus, Meta, Shell } = imports.gi;
-const Signals = imports.signals;
+const Signals = imports.misc.signals;
 
 const IBusCandidatePopup = imports.ui.ibusCandidatePopup;
 
@@ -40,8 +40,10 @@ function getIBusManager() {
     return _ibusManager;
 }
 
-var IBusManager = class {
+var IBusManager = class extends Signals.EventEmitter {
     constructor() {
+        super();
+
         IBus.init();
 
         // This is the longest we'll keep the keyboard frozen until an input
@@ -325,4 +327,3 @@ var IBusManager = class {
                 });
     }
 };
-Signals.addSignalMethods(IBusManager.prototype);