A basic javafx calculator app that supports:
        - All digits (0-9)
        - Addition, subtraction, multiplication, and division
        - Backspacing
        - Negation
        - Decimals
        - Calling back previous answer

    After run, the calculator window cannot be resized. Changing the size constants in the code will not ruin button display.

    If you are interested in adding more buttons:
        - This code accommodates for organization of new buttons
        - New buttons' icons will need to be added to the icons array in Button.java
        - New buttons' functionalities will need to be programmed individually
        - New order of assigning functions to buttons will likely be necessary