Highlight fade in SwCommentRuler

It was not implemented, though designed.
No delay was used before start fading. I don't think it flickers.

https://wiki.documentfoundation.org/Design/Whiteboards/Comments_Ruler_Control#Behavior

Change-Id: I419220aac0a9cbfae328874cf6015d9530b93784
Signed-off-by: Rodolfo Ribeiro Gomes <rodolforg@gmail.com>
Reviewed-on: https://gerrit.libreoffice.org/6706
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>