* calendar/todos.el: Revise handling of initial display and extend

* calendar/todos.el: Revise handling of initial display and extend
to top priorities file.
(todos-display-categories-first): Remove, replacing by this:
(todos-show-first): New defcustom.
(todos-first-visit): Remove, replacing by this:
(todos-visited): New variable.
(todos-absolute-file-name): Extend possible values of optional
argument to take top priorities files into account.
(todos-modes-set-3, todos-mode): Remove use of todos-first-visit.
(todos-mode-external-set): Call find-file-noselect instead of
find-buffer-visiting, since the latter may fail.
(todos-show): Revise handling of initial display using
todos-show-first and todos-visited and extend to top priorities
file.
(todos-quit): Adapt to new handling of initial display.