[FEATURE] Java-based GUI for calculator

[tommens]

Describe the new functionality you would like to see
In the current implementation, arithmetic expressions need to be build manually, from scratch. A real calculator has a graphical user interface (GUI) through which you can enter expressions by pressing the buttons of your calculator, and getting the result of evaluating your expression that will be shown on the calculator's display screen.

Additional context
To create the GUI, select and motivate the most appropriate framework for doing so. Many Java GUI frameworks are available: Oracle JavaFX (successor of Swing), Oracle Swing (successor and extension of AWT), Apache Pivot (open source framework), Eclipse SWT (also open source), ...