… the public API.

This was initially proposed by Niels De Graef in !101, and though I
still think this is much less practical for day-to-day development, it
is actually much nicer for the public part of the API. So let's use
these only in public libgimp* API only, not in core.

I actually already started to use these for some of the libgimpwidgets
classes and am now moving libgimp main classes to these macros.

* It doesn't expose the priv member (which is completely useless for
  plug-in developers, only to be used for core development).
* It forces us to never have any variable members in the public API
  (which we were doing fine so far in newest API, but it's nice to be
  enforced with these macros).
* When using G_DECLARE_FINAL_TYPE in particular, it adds flexibility as
  we can change the structure size and the members order as these are
  not exposed. And if some day, we make the class derivable with some
  signals to handle, only then will we expose the class with some
  _gimp_reserved* padding (instead of from the start when none is
  needed). Therefore we will allow for further extension far in the
  future.

Moreover most of these libgimp classes were so far not using any private
values, so we were declaring a `priv` member with a bogus contents just
"in case we needed it in future" (as we couldn't change the struct
size). So even the easiness of having a priv member was very relative
for this public API so far (unlike in core code where we actually have
much more complex interactions and using priv data all the time).