(dired-faces): New defgroup.

(dired-faces): New defgroup.
(dired-header, dired-mark, dired-marked, dired-flagged)
(dired-warning, dired-directory, dired-symlink, dired-ignored):
New faces.
(dired-header-face, dired-mark-face, dired-marked-face)
(dired-flagged-face, dired-warning-face, dired-directory-face)
(dired-symlink-face, dired-ignored-face): New face variables.
(dired-font-lock-keywords): Use them instead of font-lock faces.
Split the rule for dired marks into 3 separate rules: for marks,
marked file names and flagged file names.