The GimpDrawable abstraction is completely gone, GimpTile is now a
small struct in gimptilebackendplugin.c.
All tile handling code is now in GimpTileBackendPlugin, the backend
functions are simply calling gimp_tile_get() and gimp_tile_put()
directly.
even if we don't have private members (yet). Also make class padding 8
pointers in all headers. This commit moves nothing to private, it just
makes all headers consistent and adjusts .c files accordigly.
