[backport#16118] gui: Enable open wallet menu on setWalletController

Summary:
https://github.com/bitcoin/bitcoin/pull/16118/commits/75485ef0962a53946f17b761c4445627b07e6eff

Depends on D6169

Backport of Core PR16118

Test Plan:

ninja

I don't have access to a macOS box to check the menu is disabled while the splash screen loads, wat do

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6170