Un-revert D18150 + apply a bugfix for a MacOS crash on shutdown:
qt: Override BitcoinApplication::event() to handle QEvent::Quit
#336 introduced a regression when termination requests from a platform are not handled properly.
This PR fixes this regression. On macOS shutdown after clicking "Quit" in Dock icon menu, and during logout works again.
This concludes backport of core-gui#336 and core-gui#547