Update the Calqltr demo visuals and engine logic

Update the Calqltr demo visuals and engine logic

    - Add logic for displaying the calculation result in the best
      available precision, determined by the display width
    - Display 'ERROR' when the result cannot be displayed
    - Animate the number pad button colors to react to presses and
      visually disable them when pressing the button has no effect
    - Fix issues in calculator.js logic
    - Update documentation accordingly

Task-number: QTBUG-41253
Change-Id: Ibed7b8218ea4cd074b8f9b90d9bb4e3ea6b25ba2
Reviewed-by: Johanna Äijälä <johanna.aijala@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>