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/signals.js

@@ -0,0 +1,22 @@
+const Signals = imports.signals;
+const SignalTracker = imports.misc.signalTracker;
+
+var EventEmitter = class EventEmitter {
+    connectObject(...args) {
+        return SignalTracker.connectObject(this, ...args);
+    }
+
+    disconnectObject(...args) {
+        return SignalTracker.disconnectObject(this, ...args);
+    }
+
+    connect_object(...args) {
+        return this.connectObject(...args);
+    }
+
+    disconnect_object(...args) {
+        return this.disconnectObject(...args);
+    }
+};
+
+Signals.addSignalMethods(EventEmitter.prototype);