[backport#16224 1/5]gui: Add detailed text to BitcoinGUI::message

Summary:
Backport of Core PR16224 - part 1-5
Commit https://github.com/bitcoin/bitcoin/pull/16224/commits/23b9fa2e5ec0425980301d2eebad81e660a5ea39

Test Plan:
The added code is not yet used in this commit.
Just make sure it still compiles.
ninja

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7985