Forward native key codes to QWindowSystemInterface

Forward native key codes to QWindowSystemInterface

Using QWindowSystemInterface::handleExtendedKeyEvent() instead of
QWindowSystemInterface::handleKeyEvent(). While the former allows
us to forward the native key codes to QWindowSystemInterface, the
latter initializes them with 0, causing QKeyEvent::native*() to
return 0, instead of the actual values.

Change-Id: I596ad2e07645e091529ca514682c98d095244a73
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>