New implementation of Todo item insertion commands and key bindings.

New implementation of Todo item insertion commands and key bindings.

* calendar/todo-mode.el: New implementation of item insertion
commands and key bindings.
(todo-key-prompt): New face.
(todo-insert-item): New command.
(todo-insert-item--parameters): New defconst, replacing defvar
todo-insertion-commands-args-genlist.
(todo-insert-item--param-key-alist): New defconst, replacing
defvar todo-insertion-commands-arg-key-list.
(todo-insert-item--keyof, todo-insert-item--this-key): New defsubsts.
(todo-insert-item--argsleft, todo-insert-item--apply-args)
(todo-insert-item--next-param): New functions.
(todo-insert-item--args, todo-insert-item--argleft)
(todo-insert-item--argsleft, todo-insert-item--newargsleft):
New variables.
(todo-key-bindings-t): Change binding of "i" from
todo-insertion-map to todo-insert-item.
(todo-powerset, todo-gen-arglists, todo-insertion-commands-args)
(todo-insertion-command-name, todo-insertion-commands-names)
(todo-define-insertion-command, todo-insertion-commands)
(todo-insertion-key-bindings, todo-insertion-map): Remove.