* calendar/todos.el: Improve definitions and use of some faces.

* calendar/todos.el: Improve definitions and use of some faces.
(todos-top-priority): Use current definition of font-lock-constant-face.
(todos-diary-expired): Make default value contrast more with
default value of todos-date face.
(todos-nondiary, todos-category-string): New faces.
(todos-nondiary-face, todos-category-string-face): Corresponding
new variables.
(todos-done, todos-comment): Switch default values to go better
with default value of todos-date face.
(todos-done-sep): Change default value.
(todos-category-string-matcher-1)
(todos-category-string-matcher-2): Improve doc string.
(todos-font-lock-keywords): Use todos-nondiary-face and
todos-category-string-face.
(todos-done-separator): Propertize string with todos-done-sep face.
(todos-filtered-items-mode): Fix typo.