Use the new Fl::delete_widget() call to delete widgets in callbacks
in FLTK 1.1.6. FLTK 1.1.5 introduced a potential crash due the fact
that it could access a widget's data after its callback was executed.
Fl::delete_widget() in 1.1.6 delays the deletion until the next
Fl::wait() call. In any case, this means that we cannot use
group.clear() anymore.