Create a QCoreApplication in QtWebEngineProcess as well.

A QCoreApplication is needed in order to correctly load the
configuration from qt.conf and setup the Qt environment variables
accordingly. This is important to be able to deploy applications.

This patch creates a dummy QCoreApplication in process main to
have the environment setup correctly.

Change-Id: If536b9edf357d5925fee02f2528872101139f87e
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>