libgimp: gimp_pdb_get_data() and gimp_pdb_set_data() are made internal.

@cmyk.student is working on the van-gogh-lic plug-in right now which is the last one with gimp_get_data and gimp_set_data calls. Once he pushes, we'll be able to get rid of these 2 historical functions which are not needed anymore (we have much better with AUX arguments, or even just normal arguments to store various run variables between calls, even across sessions), by merging this MR.