Clean-up and refactoring on statement widget visitor
Improve layout
Fix IfNode observer with if statement without else keyword
Refactoring
Refactor of comparing abstract types

Implemented equals method to compare types.
user input validation fix
Moved ui related methods to main

Frame generator is not concerned with the size of the frame. This is a
UI concern which is handled by main program
Refactor of Eval value unit test

Organized package
Refactored code, deleted unused code.
Added visitor interface for statement checker
Added visitor accept method to FormStatement
Implemented visitor accept method to children of FormStatement
Beginning of StatementChecker
Renamed condition field to expr field in ConditionBlock class
Implemented functionality for StatementChecker
Refactoring
Refactored StatementChecker for some information hiding
Moved ExprTypeChecker super class to subpackage
Moved ASTNodeChecker super classes to subpackage
Refactor of unit tests

Removed Test-prefix from filenames.
Removed Test-prefix from testmethods.
Removed Statement-postfix from statement testmethods.