[backport#15153] gui: Add thread to run background activity in WalletController

Summary:
https://github.com/bitcoin/bitcoin/pull/15153/commits/be82dea23c3e6931f9ae7ab8c6a8595ae950587f

Depends on D6115

This is a partial backport of Core PR15153

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6119