Improve the code quality

[cavallium]

I didn't expect this program to become so famous, so most of the code was written badly and quickly in the past five years.
It would be necessary to improve the quality of the code by:

• improving the names of the variables
• making the code more understandable
• fixing some bad or overcomplicated patterns
  I will correct these problems of my old code after I have completed all the main components of the calculator.