* todos.el: Fixes concerning filtered items.

* todos.el: Fixes concerning filtered items.
(todos-mode-line-control): Make doc string more precise.
(todos-read-file-name): Default to current file when user hits RET
without choosing a file, to avoid creating file with empty base name.
(todos-filter-items): Rename buffer of filtered items files.
(todos-find-item): Save match data to avoid arg out of range error.
(todos-print-buffer): Fix header of filtered items buffer output.