General improvements for the Calculator App

[JingMa87]

This is your chance to start being part of the crazy world of Open Source! Go to the CalculatorApp, read through the code and see if you can contribute anything. If you have any questions, ask me and I'll mentor you. Have fun! 💯

[vharsh]

Why do the files lack a .java extension?
Are you using another compiler?

[ketanhwr]

Hi! I've opened a pull request. Can you look into it?

[JingMa87]

@vharsh I didn't upload the whole files cause of generated code for the GUI, so I only uploaded the source code.
@ketanhwr I'll have a look right now!