3D import issues when components view is closed

Some issues with 3D import when components view (item library) is closed:

• If the view is closed at project load, importable 3D types are not registered, as the notification from puppet doesn't get handled.
• Imported 3D component won't appear in "My 3D Components"
• Imported 3D component won't appear in Creat/imported models context menu in 3D view.

The last two may be related to importsChanged not getting handled by components view, but that's just a guess at this point.
The registration issue requires 3D import functionality being moved into some widgetless view. NodeInstanceView is a natural place, as the functionality uses puppet.