gui: Do not exit and re-enter main event loop during shutdown

Summary:
Relevant part of the PR description

During shutdown QApplication exits the main event loop, then re-enter again.

This PR streamlines shutdown process by removing the need to interrupt the main event loop.

qt, refactor: Allocate SendConfirmationDialog instances on heap

This change is require for the next commit.

https://github.com/bitcoin-core/gui/pull/336/commits/b4e0d2c43181ad97c15b252e95181e2c3f6c1d2a

qt: Do not exit and re-enter main event loop during shutdown

https://github.com/bitcoin-core/gui/pull/336/commits/f3a17bbe5f7d23b6ecc20e363920492b50859dad

qt, refactor: Drop intermediate BitcoinApplication::shutdownResult slot

https://github.com/bitcoin-core/gui/pull/336/commits/451ca244db8bc71ffc3cc9982d025f144cc8f3bc

For me this backport is mainly about grabbing the refactorings that makes core-gui#586 (Qt 6 readiness) more straightforward to backport

This concludes backport of core-gui#336

Depends on D18149

Test Plan:
ninja all check-all
run bitcoin-qt

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18150