* tools/pdbgen/*.pl: added 2000 to the copyrights

* tools/pdbgen/lib.pl: it's foo_pdb.[ch] now

* tools/pdbgen/pdb/drawable.pdb
* tools/pdbgen/pdb/parasite.pdb: reenabled sanity checks for the
drawable IDs and the parasite names

* app/drawable.c
* app/gimpdrawable.c: added a bunch of g_return_if_fail's, instead
of the silent check and return in many functions. This should also
be done in gimpimage.c and the like too.

* app/gimpdrawable.h
* app/layer.c: cosmetic code fix

-Yosh