signalTracker.js: Don't track "destroy" for non-ClutterActors.

The CinnamonWM class (global.window_manager) has a "destroy" signal
which maps to windows being destroyed, not its own class, but this
gets caught when using connectObject for signal handling, causing
all handlers to be released when the class emits destroy.

ClutterActors are really the only thing in Cinnamon's runtime that
will have a destroy signal, so just do an instance type check.

Author: mtwebster

js/misc/signalTracker.js

@@ -1,14 +1,19 @@
 /* exported addObjectSignalMethods */
 const GObject = imports.gi.GObject;
+const Clutter = imports.gi.Clutter;
 
 /**
  * @private
  * @param {Object} obj - an object
- * @returns {bool} - true if obj has a 'destroy' GObject signal
+ * @returns {bool} - true if obj's 'destroy' signal indicates its own lifecycle
+ *   end (i.e. obj is a Clutter.Actor or subclass).
+ *
+ * Some classes (e.g. Cinnamon.WM) declare a 'destroy' signal with
+ * domain-specific semantics unrelated to the emitter's own lifetime, so we
+ * only treat the signal as lifecycle for Clutter actors.
  */
-function _hasDestroySignal(obj) {
-    return obj instanceof GObject.Object &&
-        GObject.signal_lookup('destroy', obj);
+function _hasLifecycleDestroy(obj) {
+    return obj instanceof Clutter.Actor;
 }
 
 var TransientSignalHolder = GObject.registerClass(
@@ -20,7 +25,7 @@ class TransientSignalHolder extends GObject.Object {
     constructor(owner) {
         super();
 
-        if (_hasDestroySignal(owner))
+        if (_hasLifecycleDestroy(owner))
             owner.connectObject('destroy', () => this.destroy(), this);
     }
 
@@ -84,7 +89,7 @@ class SignalTracker {
      * @param {Object=} owner - object that owns the tracker
      */
     constructor(owner) {
-        if (_hasDestroySignal(owner))
+        if (_hasLifecycleDestroy(owner))
             this._ownerDestroyId = owner.connect_after('destroy', () => this.clear());
 
         this._owner = owner;
@@ -152,7 +157,7 @@ class SignalTracker {
      * @returns {void}
      */
     track(obj, ...handlerIds) {
-        if (_hasDestroySignal(obj))
+        if (_hasLifecycleDestroy(obj))
             this._trackDestroy(obj);
 
         this._getSignalData(obj).ownerSignals.push(...handlerIds);