gvle: fix the menubar that disappear

The Gtk::panedview does not have to be the child of Gtk::Window but
child of a Gtk::Box. (closes #165).
gvle: fix the scrollbars of the main window
gvle: fix crash when menu file/new is used

The default simulation condition (with duration and begin parameters)
missed. We need to add the default condition into the
vle::vpz::Conditions default constructor. (closes #166).