Replace GtkWidget::delete-event by GtkWindow::close-request

Replace GtkWidget::delete-event by GtkWindow::close-request

The event is not useful at all, so we are better off
with a signal that doesn't have it, and it is only
relevant on toplevel windows, so we don't need it on
GtkWidget.

With this commit, delete events no longer go through the
::event, ::delete-event, ::event-after widget signals,
but just cause the ::close-request signal on GtkWindow to be
emitted.