* calendar/todos.el: Bug fixes involving or displaying done items.

* calendar/todos.el: Bug fixes involving or displaying done items.
(todos-done-separator): Make length of long separator one less
than window-width, in order to avoid following empty line.
(todos-forward-item, todos-backward-item): Make empty line below
last todo item accessible only by invoking without a numerical
prefix argument and make the latter invocations move only to items.
(todos-insert-item): Set todos-current-todos-file after selecting
buffer, in order not to set global value.  If only done items are
shown when this command is invoked, toggle to show todo items to
avoid insertion into done items section.  When done items are
shown, protect from cancelling before setting item priority: this
also needs recentering to display all items.
(todos-set-item-priority): Only call on (not done) todo items.
Check that done items are visible not because file is widened.
Use only `any' argument of called-interactively-p.  Make sure that
giving an undone item lowest priority restores it to the todo
section.
(todos-item-done): If done items are visible on invoking this
command, keep them visible afterwards.  Stop looking for marked
items on reaching the empty line below the last todo item.