image-dired: Add new %-format specifiers for header line

image-dired: Add new %-format specifiers for header line

* lisp/image/image-dired.el
(image-dired-format-properties-string): Add new specifiers "%n", "%d",
and "%s".
(image-dired-display-properties-format): Change default format to use
above new specifiers.
(image-dired--number-of-thumbnails): New variable.
(image-dired-insert-thumbnail, image-dired-display-thumbs): Keep
track of file number when appending it into the thumbnail buffer.
(image-dired--update-header-line): Support above new format
specifiers.  Rename from 'image-dired-update-header-line' and
update callers.
* lisp/image/image-dired-dired.el
(image-dired-dired-display-properties): Support above new format
specifiers.