First Attempt at On Screen Calculator

Hi Everyone.  This is my first attempt at the On Screen Calculator.  My commits are not small, and I apologize for that Zee.  The only functionality that I see that throws an error is clicking on operators more than once before clicking on another number.  I don't think some of the edge cases I addressed were necessary, but since I'm building these applications for my portfolio I'd like it to be fairly clean.

Thank you for your help.