Instead of filling default GUI for a specific type of plug-in procedure in
fill_list(), we add 2 methods:

* fill_start() is ensured to run once (and only once) before any fill_list()
  code runs.
* fill_end() is ensured to run once (and only once) after all fill_list() ran.

This takes care of 2 kind of GUI bugs which we could have:

1. First if no explicit fill were run (i.e. neither gimp_procedure_dialog_fill()
   nor gimp_procedure_dialog_fill_list() were ever run), then the default
   interface would not be added to the dialog. Yet this case could happen when
   we don't want anything else but the default GUI (this will be the case in the
   upcoming file-wmf-load GUI).
2. Second if at the opposite, you fill several times fill functions (I hadn't
   thought of this, but noticed some already started to do this in our ported
   plug-ins), we obviously don't want the default GUI to be added several times
   either.