gimp_plug_in_manager_add_to_db() used to call the PDB in order
to properly register file procedures from pluginrc. This broke
when I moved code to gimpplugin-file.c.

Instead, add gimp_plug_in_manager_add_load_procedure() and
gimp_plug_in_manager_add_save_procedure() and call them from both
gimpplugin-file.c and gimp_plug_in_manager_add_to_db(), which also has
the nice side effect that more GimpFileProcedureGroup logic gets moved
to its place in gimppluginmanager-file.c and less magic is needed in
gimpplugin-file.c.