image-dired: Add support for PDF files (Bug#26432)

image-dired: Add support for PDF files (Bug#26432)

* lisp/image/image-dired.el
(image-dired--file-name-regexp): New function.
(image-dired-display-thumbnail-original-image): Clean up.
(image-dired-display-image): Support visiting PDF files.
* lisp/image/image-dired-dired.el
(image-dired-dired-toggle-marked-thumbs):
* lisp/image/image-dired.el (image-dired-get-thumbnail-image)
(image-dired-show-all-from-dir): Use above new function instead of
'image-file-name-regexp'.