Break sessions out of SMTK core and into plugins

As more SMTK sessions and extensions are created, it is difficult to track how modifications to SMTK core will affect plugins and extensions that are not part of the SMTK project. One step towards fixing this issue is to extract non-SMTK core components out of SMTK and into separate repositories. This way, our testing of these components will be on equal footing with the testing of other out-of-core plugins/extensions.