image-dired: Add face for flagged files

image-dired: Add face for flagged files

* lisp/image-dired.el (image-dired-thumb-mark): New defface.
(image-dired-thumb-update-marks): Mark flagged file with above new defface.
(image-dired-dired-file-marked-p)
(image-dired-thumb-file-marked-p): Add optional argument to be
used by 'image-dired-*-flagged-p' functions below.
(image-dired-dired-file-flagged-p)
(image-dired-thumb-file-flagged-p): New defuns.
(image-dired-thumb-mark): Improve definition with color classes.
(image-dired-thumb-visible-marks): Doc fix.