Added a lot of new functions - check description

- Text view starts by displaying zero
- Added opr, result, operand1 and operand2 for later use with
displaying the results
- Added IfZero() function - prevents multiple zeros if a non zero
integer is not present
- Added a working CLEAR button - clears everything and displays zero
(“0”)
- Added working number pad with numbers from 0-9 - All have IfZero()
function checking for zeros (check IfZero() functionality further up)
- Added working “+” and “-” - multiples of the same symbol cannot be
present
- Added an addition() function so we can be able to make use of
addition (WIP - needs multiple operands and not just two)
- Added an equals button that should display the result when pressing
“=” (WIP - does not currently work - crashes the window. Needs added
functionality for the other mathematical functions)