The current swap-dir management code doesn't cope well with
programs that modify the swap dir after initialization, which is
what GIMP is now doing (see issue gimp#2224.)

Move all the swap-dir management code to a new gegl-buffer-swap.c
file, and have gegl-init.c and gegl-buffer-config.c configure the
swap through this file.

The new file provides two new public functions:
gegl_buffer_swap_create_file(), which returns a unique filename for
a swap file in the current swap dir, and
gegl_buffer_swap_remove_file(), which removes and deletes a
previously-created swap file.  All swap files created with
create_file(), which hasn't been explicitly removed with
remove_file(), are automatically deleted on shutdown.  This allows
swap files to be safely deleted, even if the swap dir has been
changed since their creation.

Deprecate gegl_tile_backend_unlink_swap() in favor of the new
functions.

Use gegl_buffer_swap_{create,remove}_file() in the swap backend.