Drop gtk_widget_destroy

This is part of a larger effort to drop GtkContainer. Once GtkContainer is gone, gtk_widget_destroy() will be dangerous, since there is no generic facility anymore for GTK to clean up in the parent after destroying the widget.

This branch replaces gtk_widget_destroy() by three things:

• gtk_window_destroy() for explicitly destroying toplevels
• gtk_container_remove() or widget-specific child removal apis
• g_object_unref() for freestanding non-toplevels

Much of the work in this branch has been picked from the older wip/baedert/gtk-widget-destroy branch by Timm Baeder.