Use gimp_async_add_callback_for_object(), added in the previous
commit, instead of gimp_async_add_callback(), in cases where the
destructor of the object owning the async doesn't wait for the
async to finish.  This avoids leaking such ongoing asyncs on
shutdown, during which gimp-parallel either finishes or aborts the
asyncs: if at this point an async has any registered callbacks, an
idle source is added for running the callbacks, extending the
lifetime of the async; however, since we're not getting back into
the main loop, the idle is never run, and the async (and any
associated resources) are never freed.