Make `image-dired-thumb-(height|width)' obsolete

Make `image-dired-thumb-(height|width)' obsolete

* lisp/image/image-dired.el (image-dired-thumb-width)
(image-dired-thumb-height): Make obsolete.
(image-dired-thumb-size): Clean up and improve docstring.
* lisp/image/image-dired-external.el (image-dired-thumb-size): Use
'image-dired-thumb-size' instead of above obsolete variables.
(image-dired-create-thumb-1): Support %s format specifier for size.
(image-dired-cmd-create-thumbnail-options): Document %s format
specifier.
(image-dired--thumb-size): Rename function from
'image-dired-thumb-size' and make old name into an obsolete alias.
Update all callers.