qt: Replace deprecated QSignalMapper by lambda expressions

Summary:

The QSignalMapper class has been deprecated since Qt 5.10.
This PR replaces it by lambdas and does not change behavior.

Backport of core PR16706.

Test Plan:
With Qt >= 5.10 and <= 5.13

ninja all check

Check the warnings are gone

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7887