gui: Call setWalletActionsEnabled(true) only for the first wallet

Summary:

On master there is a bug:

• open an encrypted wallet; please note that the "Encrypt Wallet..." menu item is disabled that is expected
• then open any other wallet; note that the "Encrypt Wallet..." menu item gets enabled that is wrong

This PR fixes this bug.

This is a backport of core-gui#43

Test Plan:
ninja all check-all

Failed to reproduce the bug after this change.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10604