[backport#15450 1/5] gui: Refactor OpenWalletActivity

Summary:
https://github.com/bitcoin/bitcoin/pull/15450/commits/bc6d8a3662f0fb992073e5e80269a90a722d76e0

Partial backport of Core PR15450

depends on D7108

Test Plan:

ninja check

Open bitcoin-qt open and close wallets to make sure it works as before

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7102