signalTracker: Use "destroy" only for actors and TransientSignalHolders

The new signal tracker listens to "destroy" signals of GObjects in order to detect the end of an objects lifetime. Coincidentally, mutter uses "destroy" as a term when closing windows and notifies its plugins using a vfunc called "destroy", which ShellWM then emits as a signal.

This signal is obviously not meant for notifying about the end of ShellWMs lifetime, but the signal tracker detects it and will disconnect all the signals from ShellWM. We can't rename the "destroy" signal of ShellWM right now, so to avoid this conflict we don't use the SignalTrackers "destroy" detection magic for now and limit listening to "destroy" to ClutterActors and TransientSignalHolders.