[Qt] Restore < Qt5.6 compatibility for addAction

Summary:
PR14573 / D7892 broke Qt5.6 compatibility due to calling the lambda version of addAction that was added in Qt5.6.

Backport of Core PR14979

Test Plan:
ninja && ninja check
Run src/qt/bitcoin-qt and test all menu actions

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7894