Gtk::Builder: Make get_widget_derived() a variadic template

* gtk/src/builder.hg: get_widget_derived() can take extra parameters, which
are forwarded to the constructor of the derived class.
Inspired by a patch by Armin Burgmeier <armin@arbur.net>.
* tests/builder/main.cc: Add a parameter to the constructor of the derived
class. Bug #134161.