Clear calculator result if closed, remember result if hidden/minimized

## Problem

Calculator currently always remembers the last result and has to be cleared first when the calculator is needed again. This requires the user to perform an extra step of clearing the calculator before using it.

## Proposal

a) Clear the calculator result if it was closed. A user closing the calculator means that the user is done.
When the calculator is opened again some time later, the user will be presented with a cleared calculator.

b) Remember the calculator result if it was hidden/minimized. A user hiding the calculator means that the user is still working with the calculator.

c) Provide an option to toggle, for users who prefer the current behavior.

## Prior Art