QmlDesigner: Fix resolving of source

Using the QUrl:resolve() does not work with Qt 5.9.
Since the file url is a local file and not directory this was
weird usage of QUrl anyway.

Change-Id: I2d709bc40a404b61ea3c3edbbfd1727a6ee2111c
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>