WinRT: CoInitializeEx in event dispatcher constructor if it wasn't called before

In case of QThread autotest the application tries to create an event
dispatcher even thought CoInitializeEx was not called before. If that
is the case obtaining the timer factory fails. Call CoInitializeEx so
that the auto test can be run.

Change-Id: Ice9a43097c2e9979c24d853211d0fbdcbf48fb03
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>