Extend todo item insertion UI to item editing.

Extend todo item insertion UI to item editing.

* calendar/todo-mode.el: Reimplement item editing to have the same
basic user interface as item insertion, and make small UI and
larger internal improvements to the latter.
(todo-insert-item): Add reference to the Todo mode user manual to
the documentation string.
(todo-insert-item--basic): Rename from todo-basic-insert-item and
adjust all callers.  Change signature to combine diary and
nonmarking arguments.  Incorporate functionality of deleted item
copying command and add error checking.  Remove detailed
descriptions of the arguments from the documentation string, since
this is treated in the Todo mode user manual.
(todo-copy-item, todo-edit-multiline-item)
(todo-edit-done-item-comment, todo-edit-item-header)
(todo-edit-item-time, todo-edit-item-date-from-calendar)
(todo-edit-item-date-to-today, todo-edit-item-date-day-name)
(todo-edit-item-date-year, todo-edit-item-date-month)
(todo-edit-item-date-day, todo-edit-item-diary-nonmarking):
Remove.
(todo-edit-item): Reimplement as wrapper command for
todo-edit-item--next-key and make it distinguish done and not done
todo items.
(todo-edit-item--text): New function, replacing old command
todo-edit-item and incorporating deleted commands
todo-edit-multiline-item and todo-edit-done-item-comment.
(todo-edit-item--header): Rename from todo-basic-edit-item-header.
Use only numeric value of prefix argument.  Remove detailed
descriptions of the arguments from the documentation string, since
this is treated in the Todo mode user manual.
(todo-edit-item--diary-inclusion): New function, replacing old
command todo-edit-item-diary-inclusion and incorporating and fixing
functionality of deleted command todo-edit-item-diary-nonmarking,
making sure to remove todo-nondiary-marker when adding
diary-nonmarking-symbol.
(todo-edit-category-diary-inclusion): Make sure to delete
diary-nonmarking-symbol when adding todo-nondiary-marker.
(todo-edit-category-diary-nonmarking): Fix indentation.
(todo-insert-item--parameters): Group diary and nonmarking
parameters together.
(todo-insert-item--apply-args): Adjust to signature of
todo-insert-item--basic and incorporate copy parameter.
Make small code improvements.
(todo-insert-item--next-param): Improve prompt and adjust it to
new parameter grouping.  Remove obsolete code.
(todo-edit-item--param-key-alist)
(todo-edit-item--date-param-key-alist)
(todo-edit-done-item--param-key-alist): New defconsts.
(todo-edit-item--prompt): New variable.
(todo-edit-item--next-key): New function.
(todo-key-bindings-t): Bind "e" to todo-edit-item.  Remove
bindings of deleted commands.