Added function to delete notes

The function delete-selection will remove the note currently selected by
title-listbox from the list. The note will also be deleted from the
defined db-spec via primary key :id. The note is first deleted from the
database so that a fresh query will return the notes that the listbox
should display, though there is probably a beter way of updating the
listbox then to reset the entire contents of its model.
Register delete-button to call delete-selection

When the delete button is pressed, the delete-selection function is
called which will delete the note that is currently selected in
title-listbox.