enable/disable buttons according to min/max

Working:
- disable right button when making the number decimal would exceed max
- disable left button when the inverted number would exceed min/max

Works only for positive numbers and when both min and max are set:
- disable 0 when number is empty and min is greater than 0
- disable number button when adding the number would exceed max

Maybe we could either have defaults for min/max or merge the setMin()/setMax methods.