Use text properties for color escape highlighting in Shell mode.

Use text properties for color escape highlighting in Shell mode.

* ansi-color.el: Don't set comint-output-filter-functions; it is
now in the initial value defined in comint.el.
(ansi-color-apply-face-function): New variable.
(ansi-color-apply-on-region): Use it.
(ansi-color-apply-overlay-face): New function.

* comint.el: Require ansi-color.
(comint-output-filter-functions): Add ansi-color-process-output.

* shell.el (shell): No need to require ansi-color.
(shell-mode): Use ansi-color-apply-face-function to highlight
color escapes using font-lock-face property.