These are not usable by plug-ins anymore which should store their data between
runs as arguments or aux arguments (in case of values which should be stored
from one run to another but are not really usable for non-interactive scripts).
These are per-plug-in (not polluting the whole process space with just random
strings as identifiers which could be used by other plug-ins) and even survive
restarts of GIMP.

I still keep these functions, but only internally, as they are used to store
settings of GimpAspectPreview, GimpDrawablePreview and GimpZoomPreview across
plug-in runs. Still I changed their API to set and return a GBytes directly
(mimicking the private PDB functions' API).
Also I remove gimp_pdb_get_data_size() which is useless when exchanging GBytes
directly.

Note that the 2 functions are still exported in the library, and only not
advertized through headers (so they are not really internal, just hidden), on
purpose, because we...