Do not hardcode the font size for buttons

[probonopd]

To allow for, e.g.,

Also, the "About This Computer" screen has a button with smaller text, which is currently ignored for the same reason.

Also, we should set the fixed height for buttons only if the default font size is not changed; once that size is changed, we should not fix the button height anymore.

Reference:

• Update Calculator.app Utilities#189