Add plaintext reveal/conceal button

Modern password entry field best practices stipulate that the user should be offered the option of displaying the entered text in plaintext, as per [NIST 800-63B, section 10.1](https://pages.nist.gov/800-63-3/sp800-63b.html#sec10). This PR adds a button to `status.focusWidgets` that accomplishes this, revealing the password if it is currently obfuscated, and obfuscating it if it is not.