qt: Reset toolbar after all wallets are closed

Summary:

If the last open wallet is closed from the non-"Overview" tab, that tab remains active when a new wallet is opened

This is a backport of core#18896

Test Plan: Run bitcoin-qt, go to any "non-Overview" tab, close the wallet, open it again and make sure the Overview tab is now active.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9985