Reconstructed and backdated initial commit of todos.el.

Reconstructed and backdated initial commit of todos.el.

* calendar/todos.el: New file.  This is a copy of todo-mode.el
from revision 94343 except for the following changes: replace all
occurrences of the namespace prefix "todo-" with "todos-", delete
the defvar todo-cats (the old name of todo-categories) and its use
in todos-add-category, delete all defaliases of old command names.