Queue GC when a GObject reference is toggled down

From IRC:

ptomato[m]	meaning during a GC, the collector asks each object which other objects that it wants to hold on to
		    so if there's an entire section of the heap graph that's not connected to anything else, and not reachable from the root set, then it can be trashed all at once
		    GObjects don't work like that, there's only a reference count but no notion of who owns the reference
		    so, a JS object that's proxying a GObject is unconditionally held alive as long as the GObject has >1 references
		    this is related to the "parentless" GObjects that andyholmes has been seeing in the heap graphs
		    this must be why immediately successive GCs keep reducing the memory - each time, a JS object proxying a GObject with only one reference is trashed
		    (the GObject's one reference is owned by the JS object itself)
		    that unrefs other GObjects, and brings them down to only 1 reference
		    which makes more JS objects eligible for trashing

This is likely the cause of the not immediate memory releases that are being observated in gnome-shell#64 (closed).