core/generation: destroy the engine after the root's children

[bbedward]

Fixes intermittent crashes when quitting or reloading while an async Loader is still loading.

The root's destroyed signal fires before its children are deleted, so we were freeing the engine too early. Queue engine teardown so the children are destroyed first. JS interruption stays immediate.

This fixes the following core dump

#5  QScopedPointer<QObjectData>::get (this=<optimized out>)                 qscopedpointer.h:113
#9  doActivate<false> (sender=0x165, signal_index=7, ...)                   qobject.cpp:4271
#10 QMetaObject::activate<void, QList<QQmlError>> (sender=0x165, ...)
#11 QQmlEngine::warnings (this=0x165, _t1=...)                              moc_qqmlengine.cpp:243
#12 QQmlEnginePrivate::warning (this=0x7fd9b79a8300, error=...)             qqmlengine.cpp:1526
        q = 0x165                               
#13 QQmlIncubator::clear (this=0x7fd99566d560)                              qqmlincubator.cpp:653
        s = QQmlIncubator::Loading                
        enginePriv = 0x7fd9b79a8300
#14 QQuickLoaderPrivate::clear (this=0x7fd9b7918800)                        qquickloader.cpp:61
#15 QQuickLoader::~QQuickLoader (this=0x7fd9c7916cc0)                       qquickloader.cpp:275
#16 QQmlPrivate::QQmlElement<QQuickLoader>::~QQmlElement
#18 QObjectPrivate::deleteChildren (this=0x7fd9b79a3a00)                    qobject.cpp:2280
        i = 3                                       
#19 QObject::~QObject (this=0x7fd9b6914040)                                 qobject.cpp:1178
#20 Reloadable::~Reloadable (this=0x7fd9b6914040)                           reload.hpp:15
#22 ShellRoot::~ShellRoot                                                   shell.hpp:12
#23 QQmlPrivate::QQmlElement<ShellRoot>::~QQmlElement
#25 QObject::event (DeferredDelete)                                         qobject.cpp:1465
#29 QCoreApplicationPrivate::sendPostedEvents                               qcoreapplication.cpp:1926
#39 QCoreApplication::exec
#40 qs::launch::launch                                                      launch.cpp:325