[BUG] Gtk2::Plug does not accept calls to set_transient_for() method due to typechecking system bug

[Thierry Vignaud <tvignaud mandrakesoft com>]

  $Plug->set_transient_for(Gtk2::Gdk::Window->foreign_new($::XID));

the previous piece of code die with the following error message:

> ====================================================================================================>
> Uncaught exception from user code:                                                                 >
>         variable is not of type Gtk2::Window at /usr/lib/libDrakX/ugtk2.pm line 860.               >
>         ugtk2::new('ugtk2','Harddrake2 version 9.1.1') called at /usr/sbin/harddrake2 line 140     >
> ====================================================================================================>

however Gtk2::Plug inherit from Gtk2::Window and thus should support
set_transient_for() method.
but the typechecking system only check for a Window not for a polymorphic Window
like it should do.