Add auto save timer to save-place (bug#75837)

Add auto save timer to save-place (bug#75837)

* lisp/saveplace.el (save-place-autosave-interval):
New user option 'save-place-autosave-interval' which defaults to nil,
and has a custom :set to manage the timer.  Add
'save-place--manage-timer' to enable or cancel the timer if the mode is
enabled and 'save-place-autosave-interval' is non-nil.  Amend
'save-place-mode' to invoke save-place--manage-timer.  Add
'save-place--cancel-timer'.  Add 'save-place--autosave'.

(cherry picked from commit 1292b64216f636bacea7fedf578b373f03affdd8)