* gegl/buffer/gegl-buffer-index.h: added rev to header and tiles,
added sketch of collaborative datastructures.
* gegl/buffer/gegl-buffer-load.c: (gegl_buffer_read_header): made 
it possible to call with offset == NULL
* gegl/buffer/gegl-buffer-save.c: (gegl_tile_entry_new),
(gegl_tile_entry_destroy): do not use slice allocations for entries.
* gegl/buffer/gegl-tile-backend-file.c: (file_entry_destroy),
(get_tile), (set_tile), (flush), (load_index), (file_changed),
(gegl_tile_backend_file_constructor): monitor buffer for changes and
reload index if revision in header has changed.
* gegl/buffer/gegl-tile-backend.h: added private pointer to storage a
backend is member of, allows the backend to send messages through the
storage's bus.
* gegl/buffer/gegl-tile-handler-cache.c: (dispose), (command),
(gegl_tile_handler_cache_invalidate): remove tiles from cache when
they've changed on disk.
* gegl/buffer/gegl-tile-source.h: added macro for
gegl_tile_source_invalidated
* gegl/buffer/gegl-tile-storage.c: (gegl_tile_storage_constructor):
store pointer to storage in backend.

svn path=/trunk/; revision=2250