QmlDesigner.PropertyEditor: Fix FileResourcesModel

The FileResourcesModel did not work reliable, because the filters
could be set after the modelNodeBackendProperty was set.
Now we properly update the model if the filters change.

Task-number: QTCREATORBUG-12537
Change-Id: I0409531ea7ff23e671b4694605a0c8d35f0be6eb
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>