QmlJSObserver: Fix crash when adding an element to some .qml files

Make sure that the 'artifical' URL of the item created actually is
absolute (starts with a file:///). Otherwise an Assert will be hit
when the imports include e.g. a .js file, and the Engine tries to
load it.

Task-number: QTCREATORBUG-2644
Reviewed-by: Olivier Goffart