[gui] intro: inform caller if intro was shown

Summary:
Partial backport of Core PR16714 - part 3 of 5
Commit 1bbc49d2078ee53488e214d00eb47462687b05c5
Depends on D7953

Test Plan:
ninja && ninja check

Start src/qt/bitcoin-qt -regtest

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7954