calc example doesn't use button widgets

Nobody has claimed this yet.

The calc example doesn't use `Button` or a similar widget that is semantically a button. Instead, it adds a click handler to the label (or actually, the label's container). This is a problem for accessibility, especially when taking keyboard focus into account (see #2095). I know why calc doesn't use the `Button` widget; `Button` brings its own styling, which isn't the desired appearance for calc. I suggest adding a widget called something like `CustomButton` that gives the caller total control over the appearance, as they have now with a clickable label, but implements the correct accessibility and keyboard focus behavior.

Start with the calc example and the existing Button widget, then read issue #2095 for the keyboard-focus context. Compare the current clickable label or its container with the proposed CustomButton behavior. Done means the calc example uses a semantically button-like widget while retaining its custom appearance and correct accessibility and keyboard-focus behavior.

Written by the indexing model from the issue text.