GimpObjectQueue implements a queue of GimpObjects.  It derives from
GimpSubProgress, and hence can be used as a GimpProgress object.
It keeps track of the total memsize of the objects that were
pushed-to and popped-from the queue, and uses these numbers to set
the corresponding subrange of the progress object when an object is
popped.

This provides an easy way to perform an operation on a set of
objects, correctly reporting progress based on the relative sizes
of the objects, which is assumed to be a good estimate of the
relative cost of processing each object.