GimpAsyncSet represents a dynamic set of running GimpAsync objects.
The objects are automatically removed from the set once they're
synced.

GimpAsyncSet implements the GimpWaitable and GimpCancelable
interfaces, allowing the entire set to be waited-on or canceled.

Additionally, GimpAsyncSet provides an "empty" property, which
indicates whether the set is empty or not.  This allows responding
to the completion of all the GimpAsync objects through the set's
"notify::empty" signal, or drive UI changes through property
bindings.