[backport#15195] gui: Add closeWallet to WalletController

Summary:
https://github.com/bitcoin/bitcoin/pull/15195/commits/f77ba34313768fde9b813243dcd408f9755b0418

Depends on D6167

Partial backport of Core PR15195

Test Plan:

ninja

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6168