VariableTable: Fix signature of the SetVariable method
StatementElement: Add the GetVaue method stub implementation
Implement GetValue method in the Variable and Literal classes
Add constructor with string argument to StatementElement and subclasses
Use constructor of StatementElement class with string argument
StatementElement: Remove the deprecated Init method
Implement literal and variables initialization in variable table
VariableTable: Fix type of the VariableMap container
VariableTable: Add the SetValue and GetValue methods