Make internal data objects non-renamable, even if they're writable,
through gimp_data_is_name_editable().  Currently, the only such
object is the custom gradient.

Prevent changing the name of non-renamable data by making the name
entry of GimpDataEditor non-editable whenever
gimp_viewable_is_name_editable() is FALSE, even if the data is
otherwise editable.

Prevent the vairous PDB -rename() functions from renaming non-
renamable data, by adding a GimpPDBDataAccess flags type,
specifying the desired access mode for the data -- any combination
of READ, WRITE, and RENAME -- and replacing the 'writable'
parameter of the gimp_pdb_get_foo() functions with an 'access'
parameter.  Change the various .pdb files to use READ where they'd
used FALSE, and WRITE where they'd used TRUE; use RENAME, isntead
of WRITE, in the -rename() functions.