Add support for "bare" (un-backed) session restores.

[vibraphone]

It should be possible for an application to import JSON corresponding to a session backed by a modeling kernel without restarting that modeling kernel (using the "native" kernel). This would allow markup of modeling entities without requiring access to the solid modeler.

The work required to support this use case is to create tests which verify that

• the proper session type ("native") is used in this case,
• unavailable modeling operations are not restored as part of the session
• modified state saved afterwards includes the original metadata so that further restorations using the original modeling kernel work (so that, for example, meshing or further modeling operations can be performed).