ibus: Return correct scan code to windows

ibus keycode is qt scancode - 8 and needs to be returned to app windows
as the right value.
Use QWindowSystemInterface::handleExtendedKeyEvent() instead of
QCoreApplication::sendEvent() in qtbase 5.6 to follow
QXcbKeyboard::handleKeyEvent().
Also qApp->focusObject() returns QMdiChild which is a QWidget and 
that is not a QWindow.

Task-number: QTBUG-47833
Change-Id: Ie634a86d7790d8093cfca2dc8666f726844ae654
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>