* todos.el: Fixes and improvements related to item relocation.

* todos.el: Fixes and improvements related to item relocation.
(todos-category-completions): On ensuring proper category display,
if done items in category are visible, keep them visible.
(todos-set-item-priority): Keep top of category in view while
setting priority.
(todos-move-item): Restore prevention of moving within the same
category.  Move done items to top, not end, of done items section.
When user quits before setting priority, make sure to return to
starting file.
(todos-item-undo): Partly reimplement along lines of
todos-move-item, adding highlighting of single item as undo
target, setting priority of multiple undone items, improving
handling of quitting before setting priority.
(todos-unarchive-items): Restore items to top, not end, of
category's done section.