Remove the "independent" parameter of gimp_parallel_run_async(),
and have the function always execute the passed callback in the
shared async thread-pool.

Add a new gimp_parallel_run_async_full() function, taking, in
addition to a callback and a data pointer:

  - A priority value, controlling the priority of the callback in
    the async thread-pool queue.  0 is the default priority (used
    by gimp_parallel_run_async()), negative values have higher
    priority, and positive values have lower priority.

  - A destructor function for the data pointer.  This function is
    called to free the user data in case the async operation is
    canceled before execution of the callback function begins, and
    the operation is dropped from the queue and aborted without
    executing the callback.  Note that if the callback *is*
    executed, the destructor is *not* used -- it's the callback's
    responsibility to free/recycle the user data.

Add a separate gimp_parallel_run_async_independent() function,
taking the same parameters, and executing the passed callback in
an independent thread, rather than the thread pool.  This function
doesn't take a priority value or a destructor (and there's no
corresponding "_full()" variant that does), since they're pointless
for independent threads.

Adapt the rest of the code to the changes.

(cherry picked from commit b74e600c)