gui: disable File->CreateWallet during startup

Summary:

Same as PR16118. Early calls to Create Wallet will crash bitcoin-qt.

This PR disables menus actions at startup and enables them again later
in BitcoinGUI::setWalletController.
I could not test that the menu is disabled, because on my Ubuntu machine
I don't have access to these menus until after the startup rescan is finished.
AFAIK, these menus are visible on Windows (through the systray icon) or Mac as
soon as the program is started, while the program is still showing the startup screen.

This is a backport of Core PR17695

Test Plan: Build and run bitcoin-qt, and make sure the menu still works after startup rescan is completed.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8302