Add an "async" field to the dashboard's "misc" group, showing the
number of async operations currently in the "running" state (i.e.,
all those GimpAsync objects for which gimp_async_finish[_full]() or
gimp_async_abort() haven't been called yet).
Have GimpAsync implement the GimpWaitable and GimpCancelable
interfaces, added in the previous two commits, instead of providing
its own public version of the corresponding functions.
Add gimp_async_cancel_and_wait() as a convenience function for both
canceling an async operation, and waiting for it to complete.
Adapt the rest of the code to the change.
Improve the formalism of a GimpAsync object being "sycned"
(previously referred to as the main thread being "synced" with the
async thread), by both providing a gimp_async_is_synced() function,
separate from gimp_async_is_stopped(), and by improving the type
and function descriptions.
Make sure all previously added callbacks have been called after a
call to gimp_async_wait[_until](), even if these functions are
called from within a callback.
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...
