[notify] merge gobjectnotifyqueue.c into gobject.c

[notify] merge gobjectnotifyqueue.c into gobject.c

This was done as a separate file before, and #include brought it into
gobject.c.  That's a bit mad, so stop doing that.

Unfortunately, the insanity steps up a level: gobjectnotifyqueue.c is
installed in the public include dir, so we can't just get rid of it
entirely.