gui: Shortcut to close ModalOverlay

Summary:
Set a shortcut to close the modal overlay in bitcoin-qt with the Esc key.
The rationale is that it makes testing easier, when you need to start the application multiple times in a row.

This is a backport of Core PR17998

Test Plan: ninja && src/qt/bitcoin-qt

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8689