closing a window with transfers causes warnings

[gue5t gue5t]

With latest git midori, downloading a file and then closing the window in which it was downloaded results in the following series of warnings:

(midori4:11132): Gtk-CRITICAL **: IA__gtk_widget_destroy: assertion `GTK_IS_WIDGET (widget)' failed

(midori4:11132): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `GtkWidget'

(midori4:11132): Gtk-CRITICAL **: IA__gtk_widget_hide: assertion `GTK_IS_WIDGET (widget)' failed

This is because midori_transferbar_info_free runs after the widgets associated with a transfer have been destroyed in the window closing. The attached patch simplifies the logic a bit and avoids this scenario by ensuring that widget destruction is not performed from code that frees the non-UI data structures.