(skip) various variable length functions (va_list functions are not
introspectable anyway, but adding manual annotations shows we've have
been through them).

Also (skip) gimp_dialogs_show_help_button even though it could be
introspected without any problem. Yet it's written that's an
internal-only function, so let's forbid bindings from using it.

Finally fix gimp_dialog_add_button() introspection by properly including
Gtk-3.0 introspected library so that g-ir-* knows the GtkWidget type.