[backport#15195] gui: Add close wallet action

Summary:
https://github.com/bitcoin/bitcoin/pull/15195/commits/94086fb59d1a05eb9a2960c2aae242e49c0a9103

Depends on D6168

Completes backport of Core PR15195

Test Plan:

ninja
./src/qt/bitcoin-qt -regtest

close [default wallet], open it again, UI behaves as expected

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6169