refactor: Add handleNotifications method to wallet

Summary:
Further stylistic cleanups in touched files:

• Sort the includes
• Wrap long single-line constructors into multiple lines

This is a partial backport of Core PR15779 : https://github.com/bitcoin/bitcoin/pull/15779/commits/fad7c33342cb51b310a7dd372bfa675df8810367

Depends on D6236

Test Plan:

ninja all check-all
./src/bench/bitcoin-bench

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6239