Partially remove exiftool dependency from image-dired.el

Partially remove exiftool dependency from image-dired.el

* lisp/image-dired.el (exif): Require.
(image-dired-cmd-read-exif-data-program)
(image-dired-cmd-read-exif-data-options)
(image-dired-get-exif-data): Make obsolete in favour of using
exif.el.  This removes a dependency on external exiftool for some
operations.
(image-dired-get-exif-file-name)
(image-dired-thumbnail-set-image-description): Use exif.el
functions instead of exiftool.
* lisp/image/exif.el (exif-tag-alist): Add description and
copyright fields.
* test/lisp/image-dired-tests.el: New file.