Slim TableTable: manage variable assignment in VariableStore

Variable set ($X=) was not possible on a TableTable.
This pull request propose a simple solution to manage them.

Note that I don't understand why there is two different stores (without any link between them) for symbols values: SlimTestContext.symbols and SlimExecutionContext.variables

If I get it right, this creates a new instance of TableTableExtract on the Executing side of Slim, meaning that you need the TableTableExtract class in the class path. It also means, that every Slim implementation in other languages needs to define a class fitnesse.testsystems.slim.tables.TableTableExtractSymbol (sic! with the same package/namespace). Which is a protocol change.
Since TableTableExtractSymbol is on the Slim execution side, it should rather be under fitnesse.slim.