feat(main...GUI): Created a GUI for the War Games Application.

What are the key features of this change

Implementation of GUI with MVC classes for using the War Games Application. The front of the GUI is an fxml-file made by SceneBuilder. Refactoring of backend classes are also included in this merge.

Size of change: Large

List changes with class/type-of-changes

• WarGamesController/feat
• WarGamesModel/feat
• WarGamesApp/feat
• ArmyEmptyOfUnitsException/feat
• Army/refactor
• Battle/refactor
• FileHandler/feat

Checklist

• Javadoc
• Tests
• Build tool test passed