[backport#15153] gui: Add OpenWalletActivity

Summary:
https://github.com/bitcoin/bitcoin/pull/15153/commits/8847cdaaaeb45c1ddee89f43ac4b8fafb20e5c0d

Depends on D6121

This is a partial backport of Core PR15153

Test Plan:

as D6115

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6128