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.