Disable the main window toolbar when the modal overlay is shown

Summary:

qt, refactor: Cleanup ModalOverlay slots

qt: Disable toolbar when overlay is shown

This is a backport of core-gui#30

Test Plan: `bitcoin-qt --regtest

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9994