* undone/main.scm (get-field-value): Allow specification of default
return if no key is found.
(next-id): New function.
(add): Add `id' to the todo item.
* undone/main.scm (get-field): Only get a cons cell, don't get its
cdr.
(get-field-value): New macro.
(define-view): Add filtered list syntax, which filters items that
don't have the needed properties. Filter results before looking
through them.
* undone/main.scm (todo-list-file): Check to see if we should use a
file in XDG_DATA_HOME or HOME.
(mkdirs): New function.
(save): Ensure the directory tree exists by calling `mkdirs' first.
