[Libreoffice] SmNodeToTextVisitor Fixes

[Luke Dixon]

Hello Jonas and list,

I started looking at the SmNodeToTextVisitor.

There were a few problems I noticed.
1. The Greek and special characters beginning with a % get changed to
comments (%%) and then lost.
2. Stacks get ## inserted instead of # for rows.
3. Some of the subs & sups seemed to need extra brackets when used with
an operator node (for example int or sum).

I also added something I spotted in SmNode::CreateTextFromNode to the
NodeToTextVisitor for SmExpressionNodes which makes it so that not so
many brackets are added.

I also have a very small patch which removes a couple of things from the
todo list (maybe it ought to be an unnumbered list, so all the numbers
don't need changing each time).

Regards,
Luke