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, however, 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.

Since we cannot know how many more wrapped GObjects are going
be marked for garbage collection after the owner is destroyed,
always queue a garbage collection when a toggle reference goes
down.

Issue: #140