Slim Query/Table Table: manage variables assignment in VariableStore (Version 2)

This is idential to pull request 525 from Antoine
Only one line is different compared to the code from Antoine. (if I didn't missed anything). But this implementation  needs no change in the protocol messages and therfore less code. Just one additional library functions must be availbale in the slim clients.

I did this pull request quickly to demonstrate the design. The new suite tests from Antoine pass. I did no further testing.
Up to Amolenaar to decide which design is the better fit for FitNesse.

I am surprised that this pull has more additions and more deletions than Antoines request, as I only took files from him (but not all). Maybe I should have syncronised to the latest FitNesse version before creating the pull requst? 

How can I see the conflicts this request creates?