Document how to process deferred deletes in a plugin-scenario

When Qt is not driving the event dispatching, the event loop level
of the thread will stay 0. As a result, we will not automatically
bump the deferred delete event's scope level to 1 when the deferred
delete happens from a code path that doesn't raise the scope level
via QScopedScopeLevelCounter, leaving the event with loop and scope
level 0.

As we only process these 0+0 deferred delete events automatically from
QCoreApplicationPrivate::execCleanup(), which is not going to be called
when the app doesn't call QCoreAppliction::exec(), we were failing to
process the deferred deletes both during the application's runtime,
as well as on shutdown.

Task-number: QTBUG-120124
Fixes: QTBUG-117975
Pick-to: 6.5
Change-Id: I2b8accb432517a48370923ee2d760f8e2e4a8055
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit 7fac2fac9a512cef2d5cdefd9837d76f8be7de05)
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/kernel/qobject.cpp

@@ -2394,6 +2394,21 @@ void QObject::removeEventFilter(QObject *obj)
     event loop was still running: the Qt event loop will delete those objects
     as soon as the new nested event loop starts.
 
+    In situations where Qt is not driving the event dispatcher via e.g.
+    QCoreApplication::exec() or QEventLoop::exec(), deferred deletes
+    will not be processed automatically. To ensure deferred deletion in
+    this scenario, the following workaround can be used:
+
+    \code
+    const auto *eventDispatcher = QThread::currentThread()->eventDispatcher();
+    QObject::connect(eventDispatcher, &QAbstractEventDispatcher::aboutToBlock,
+        QThread::currentThread(), []{
+            if (QThread::currentThread()->loopLevel() == 0)
+                QCoreApplication::sendPostedEvents(nullptr, QEvent::DeferredDelete);
+        }
+    );
+    \endcode
+
     \note It is safe to call this function more than once; when the
     first deferred deletion event is delivered, any pending events for the
     object are removed from the event queue.