Merge #586: Getting ready to Qt 6 (6/n). Replace QCoreApplication::quit() with QCoreApplication::exit(0)

252f363 qt: Replace `QCoreApplication::quit()` with `QCoreApplication::exit(0)` (Hennadii Stepanov)

Pull request description:

  ### Qt 5:
   - no behavior change.

  See https://code.qt.io/cgit/qt/qtbase.git/tree/src/corelib/kernel/qcoreapplication.cpp?h=5.15#n2012:
  ```cpp
  void QCoreApplication::quit()
  {
      exit(0);
  }
  ```

  ### Qt 6:
   - this change avoids sending a duplicated `QEvent::Quit`

  We use `QEvent::Quit` to [handle](#547) macOS dock menu events. Qt 6 uses `QEvent::Quit` more [widely](qt/qtbase@89f7a27). We do not want a duplicated `QEvent::Quit` which fires `Assert(node.args);` in the [`Shutdown()`](https://github.com/bitcoin-core/gui/blob/d1b3dfb275fd98e37cfe8a0f7cea7d03595af2e8/src/init.cpp#L200) function.

ACKs for top commit:
  promag:
    Code review ACK 252f363.

Tree-SHA512: 6a04cbcf523c0375158a59b29afadf18da99738c8db8b8728f99319a8cdc10806d2f06dc5a7d3b8b0e1a5f1711be778a75d4ecdefef7cf66e26ae2848f7f57db

Author: hebasto

src/qt/bitcoin.cpp

@@ -308,7 +308,9 @@ void BitcoinApplication::startThread()
 
     /*  communication to and from thread */
     connect(&m_executor.value(), &InitExecutor::initializeResult, this, &BitcoinApplication::initializeResult);
-    connect(&m_executor.value(), &InitExecutor::shutdownResult, this, &QCoreApplication::quit);
+    connect(&m_executor.value(), &InitExecutor::shutdownResult, this, [] {
+        QCoreApplication::exit(0);
+    });
     connect(&m_executor.value(), &InitExecutor::runawayException, this, &BitcoinApplication::handleRunawayException);
     connect(this, &BitcoinApplication::requestedInitialize, &m_executor.value(), &InitExecutor::initialize);
     connect(this, &BitcoinApplication::requestedShutdown, &m_executor.value(), &InitExecutor::shutdown);

src/qt/test/apptests.cpp

@@ -119,6 +119,6 @@ AppTests::HandleCallback::~HandleCallback()
     assert(it != callbacks.end());
     callbacks.erase(it);
     if (callbacks.empty()) {
-        m_app_tests.m_app.quit();
+        m_app_tests.m_app.exit(0);
     }
 }