* app/pdb/gimppdb-utils.[ch]: add "gboolean writable" to
  gimp_pdb_layer_is_text_layer() because that's called on
  all text layers anyway.

* tools/pdbgen/pdb/text_layer.pdb
* tools/pdbgen/pdb/vectors.pdb: pass TRUE if we want to modify the layer.

* app/pdb/text-layer-cmds.c
* app/pdb/vectors-cmds.c: regenerated.