Resurrect Widget::can_default() and can_focus()

* gtk/src/widget.{ccg,hg} (Gtk::Widget::can_default): Wrap using
_WRAP_METHOD(), thereby reintroducing the accidentally removed
method implementation.
(can_focus): Use _WRAP_METHOD() to bring back both the declaration
and the implementation of this accidentally removed method.
(get_allocation): Properly implement the accessor in terms of
gtk_widget_get_allocation().