qt: Add BitcoinUnits::formatWithPrivacy() function

Summary:

This PR allows to hide/reveal values on the Overviewpage by checking/unchecking Menu->Settings-> Mask Values

This is a backport of core#16432 [1/2]
https://github.com/bitcoin/bitcoin/pull/16432/commits/73d8ef72742ab9193e9e95158b26176bfaab3f66

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9919