applicationwindow: fix leak of help_overlay

applicationwindow: fix leak of help_overlay

> Due to Gtk+ keeping a reference to the window internally,
> gtk_window_new() does not return a reference to the caller.
> To delete a GtkWindow, call gtk_widget_destroy().

Caller(s) aren't expecting a need to delete help_overlay themselves
once they've installed it.  (E.g. see gtk_application_window_added()).

I didn't notice any direct precedents, but there's a parallel in the
current implementation of gtk_container_destroy() which uses
gtk_widget_destroy() on any added widget.

This avoids leaking 100s of kB per window, when I tested nautilus.

https://bugzilla.gnome.org/show_bug.cgi?id=772859