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.