The GeglStats object provides a set of read-only properties, aimed
at monitoring GEGL statistics.  The singleton GeglStats instance is
returned by gegl_stats(), in a similar fashion to gegl_config().

The following properties are currently implemented:

  - cache-total:  The total size of the data in the cache.

  - swap-total:  The total size of the data in the swap.

  - swap-file-size:  The size of the swap file.

  - swap-busy:  Whether there is work queued for the swap.