[sldev] Patch - Line history for chat lines

[Zi Ree]

Hi Second Life Developers!

When the viewer first became open source I wrote a small patch for it to 
implement a chat line history, so you can recall your previously written 
lines. Now with the SVN repository in place I thought it would be a good idea 
to adapt it to the new code base and put it up on the Jira for you to review 
and use.

Jira entry: https://jira.secondlife.com/browse/VWR-671

Feel free to change things if there's something wrong in the patch ;) I will 
probably adapt my "cursor column remember" patch soon, too.

Yours,
Zi!