VR keyboard doesn't work with overlay windows

[ksuprynowicz]

Overlay windows do not get events from VR keyboard, and also don't get Edit->Paste

Part of VR keyboard that injects events:

        QKeyEvent* pressEvent = new QKeyEvent(QEvent::KeyPress, scanCode, Qt::NoModifier, keyString);
        QKeyEvent* releaseEvent = new QKeyEvent(QEvent::KeyRelease, scanCode, Qt::NoModifier, keyString);
        QCoreApplication::postEvent(QCoreApplication::instance(), pressEvent);
        QCoreApplication::postEvent(QCoreApplication::instance(), releaseEvent);

Fixing this is needed for #961

[ksuprynowicz]

Maybe void OffscreenQmlSurface::synthesizeKeyPress(QString key, QObject* targetOverride) could help?