AW: [gtkmm] Porting guide additions

["Meier, Elke" <elke meier sysde eads net>]

> > o change Gtk::Widget::set_usize() to Gtk::Widget::set_size_request()
> 
> We already recommend that people use set_default_size() 
> instead. Can you
> give us more information, ideally linking to GTK+ documentation?
> > 

set_default_size() is fine for windows, but for entry widgets 
(and others) it does not compile. In fact, for windows the documentation
states that set_default_size() should be used.
(
http://gtkmm.sourceforge.net/gtkmm2/reference/html/classGtk_1_1Widget.html#a
46 )