workspaceThumbnail: Destroy thumbnails on destruction

Often, when using multiple monitors, the thumbnails box may be
destroyed before the 'hidden' signal of the Overview is emitted,
which leaves a bunch of lingering signal connections to the
workspace manager.

Call this._destroyThumbnails() in the _onDestroy handler, which
disconnects from workspace manager. This fixes the some of the log
spam that GNOME Shell produces with backtraces of destroyed actors.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1860>

js/ui/workspaceThumbnail.js

@@ -720,6 +720,7 @@ var ThumbnailsBox = GObject.registerClass({
     }
 
     _onDestroy() {
+        this._destroyThumbnails();
         this._unqueueUpdateStates();
 
         if (this._scrollValueId)