Here's a patch that adds a "Destroyable" mixin and aggressively disconnects
signals at shutdown. To my knowledge it doesn't actually fix any memory
leaks (since PyGObject hooks into the Python cycle collector to trace
signals), but it:

 - Reduces greatly the amount of cyclic garbage that needs to be collected
 - Makes the output of gc.get_referrers() on a leaked object more understandable
 - May actually fix some other types of memory leaks that otherwise wouldn't 
   be traceable (I can't think of why this would be the case - if there's
   a cycle that isn't traceable, it's a cycle that isn't traceable no matter
   how much other stuff references it, but I seemed to be seeing this in
   practice.)

Filed for contemplation, since this could also be seen as complicated things
unnecessarily. (I also don't really like the do_destroy name I used for the
implementation of destructions, since GtkWidget.do_destroy() must not be
implemented since widget->destroy() is called at finalization time after the
proxy is finalized.)