Make comint use text properties for highlighting instead of overlays:

Make comint use text properties for highlighting instead of overlays:
(comint-last-output-overlay): Variable removed.
(comint-send-input, comint-output-filter): Use text properties instead
of overlays.
(comint-insert-clicked-input): Rewrite to work with text properties as
well as overlays.
(comint-snapshot-last-prompt): Snapshot using text properties.
(comint-get-old-input-default, comint-extract-string): Don't copy text
properties.