It follows the directions from design team available at
https://wiki.documentfoundation.org/Design/Whiteboards/Comments_Ruler_Control.
It misses some fancy stuff though (like animated/delayed highlight and fading border).
Some work should be done with (svtools) Ruler also for a cleaner implementation.

Change-Id: Ib065043d05fe56fbfe6d00e0bb654966f046129b
Signed-off-by: Rodolfo Ribeiro Gomes <rodolforg@gmail.com>