It's more valuable to have the raw text data of the html instead of
having it locked up in images. If the comments are exported as
images, then it makes it almost impossible to discern what comment
belongs to what cell when viewing the html file itself.

This is done by rendering the comments in their own comment tag
(to make it still easy to extract from scripts that modify html)
that is shown to the user. This element is initially not displayed
until the user hovers over the comment indicator

The comment indicator is denoted by a 0.5em by 0.5em red square with
a black border. The notes will always appear on a pale yellow
background with black text when the indicator is hovered over.

Change-Id: I01a3dfd77ec54566e64b196b8df3309ea941ad4c
Reviewed-on: https://gerrit.libreoffice.org/18837Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>