In gimp-utils, add new gimp_idle_run_async() and
gimp_idle_run_async_full() functions, having the same signature as
gimp_parallel_run_async[_full](), but running the async function in
an idle source on the main thread, instead of in a separate thread.

The 'priority' parameter of gimp_idle_run_async_full() specifies
the idle-source priority.