COMP: Remove direct dependency on qSlicerSceneBundleIO

Instead of instantiating 'qSlicerSceneBundleIO' to load MRB file, this
commit updates the code to use the 'loadNodes' method.

This commit also allow the extension to be complied against a version
of Slicer providing either "qSlicerSceneBundleIO" or "qSlicerSceneBundleReader"