Support 3MF component path

[mikedh]

For some reason in a larger 3MF file the memory-free behavior was deadlocking somehow (maybe playing poorly with the events=('end',) somehow?) and turning a 5s load into a repeatably 1000s load. I removed the following block and it went away:

        # free resources
        obj.clear()
        obj.getparent().remove(obj)
        del obj

This also adds support for the <component path="file-path-in-archive ... mechanism which should fix #2294.