signalTracker: Explicitly register destroyable types

We currently assume that any '::destroy' signal on a GObject type has the semantics of the ClutterActor/GtkWidget signal, and should therefore result in all signals being disconnected.

But we already have a case where the assumption doesn't hold: ShellWM uses '::destroy' for the closing animation of windows, and the ShellWM object itself remains very valid after the emission.

So rather than making assumptions about '::destroy', check objects against a list of destroyable types that are explicitly registered as such.