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/oVirt.js

@@ -2,7 +2,6 @@
 /* exported getOVirtCredentialsManager */
 
 const Gio = imports.gi.Gio;
-const Signals = imports.signals;
 const Credential = imports.gdm.credentialManager;
 
 var SERVICE_NAME = 'gdm-ovirtcred';
@@ -43,7 +42,6 @@ var OVirtCredentialsManager = class OVirtCredentialsManager extends Credential.C
             });
     }
 };
-Signals.addSignalMethods(OVirtCredentialsManager.prototype);
 
 function getOVirtCredentialsManager() {
     if (!_oVirtCredentialsManager)