signalTracker: Provide monkey-patching for (dis)connectObject()

The module exports a `addObjectSignalMethods()` method that extends
the provided prototype with `connectObject()` and `disconnectObject()`
methods.

In its simplest form, `connectObject()` looks like the regular
`connect()` method, except for an additional parameter:

```js
    this._button.connectObject('clicked',
        () => this._onButtonClicked(), this);
```

The additional object can be used to disconnect all handlers on the
instance that were connected with that object, similar to
`g_signal_handlers_disconnect_by_data()` (which cannot be used
from introspection).

For objects that are subclasses of Clutter.Actor, that will happen
automatically when the actor is destroyed, similar to
`g_signal_connect_object()`.

Finally, `connectObject()` allows to conveniently connect multiple
signals at once, similar to `g_object_connect()`:

```js
    this._toggleButton.connect(
        'clicked', () => this._onClicked(),
        'notify::checked', () => this._onChecked(), this);
```

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1953>

tests/unit/signalTracker.js

@@ -0,0 +1,79 @@
+// -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
+
+// Test cases for version comparison
+
+const { GObject } = imports.gi;
+
+const JsUnit = imports.jsUnit;
+const Signals = imports.signals;
+
+const Environment = imports.ui.environment;
+Environment.init();
+
+const Destroyable = GObject.registerClass({
+    Signals: { 'destroy': {} },
+}, class Destroyable extends GObject.Object {});
+
+class PlainEmitter {}
+Signals.addSignalMethods(PlainEmitter.prototype);
+
+const GObjectEmitter = GObject.registerClass({
+    Signals: { 'signal': {} },
+}, class GObjectEmitter extends Destroyable {});
+
+const emitter1 = new PlainEmitter();
+const emitter2 = new GObjectEmitter();
+
+const tracked1 = new Destroyable();
+const tracked2 = {};
+
+let count = 0;
+const handler = () => count++;
+
+emitter1.connectObject('signal', handler, tracked1);
+emitter2.connectObject('signal', handler, tracked1);
+
+emitter1.connectObject('signal', handler, tracked2);
+emitter2.connectObject('signal', handler, tracked2);
+
+JsUnit.assertEquals(count, 0);
+
+emitter1.emit('signal');
+emitter2.emit('signal');
+
+JsUnit.assertEquals(count, 4);
+
+tracked1.emit('destroy');
+
+emitter1.emit('signal');
+emitter2.emit('signal');
+
+JsUnit.assertEquals(count, 6);
+
+emitter1.disconnectObject(tracked2);
+emitter2.emit('destroy');
+
+emitter1.emit('signal');
+emitter2.emit('signal');
+
+JsUnit.assertEquals(count, 6);
+
+emitter1.connectObject(
+    'signal', handler,
+    'signal', handler, GObject.ConnectFlags.AFTER,
+    tracked1);
+emitter2.connectObject(
+    'signal', handler,
+    'signal', handler, GObject.ConnectFlags.AFTER,
+    tracked1);
+
+emitter1.emit('signal');
+emitter2.emit('signal');
+
+JsUnit.assertEquals(count, 10);
+
+tracked1.emit('destroy');
+emitter1.emit('signal');
+emitter2.emit('signal');
+
+JsUnit.assertEquals(count, 10);