GimpAsync represents an asynchronous task, running without blocking
the main thread. It provides functions that allow waiting for
completion, queueing completion callbacks, and requesting
calcelation. See the code for more details.
The interface and the implementation are intentionally limited for
the time being, to keep things simple, and are geared toward
asynchronous tasks executed on a separate thread (see the next
commit). The public interface is relatively general, however, and
we may extend the implementation to support other kinds of tasks at
some point, who knows...
