refactored updateMiniDisplay and updateDisplay into smaller simpler methods
updated backspacePressed method to reset bool values
updated backspacePressed method to make it more user-friendly.
when user backspaces all the digits but leaves a zero, then the state will be reset (as though everything was cleared)
clearDisplay simplified to display nothing, rather than 0

updated backspacePressed method to simplify the case where user backspaces everything.
if a zero remains on screen, user can backspace it or continue typing