Add display hiding

[bentoogood]

Users are likely to want to modify what objects are visible in the 3d viewer whilst interacting with the application, without constructing and maintaining graph branches to achieve this.

suggested functionality:

• to be able to isolate the currently selected objects.
• to globally invert the display state of all objects.
• to be able to set the display state of objects in the SceneHierarchy editor (possibly with hotkeys or with toggle buttons drawn in the editor)
• to be able to see hidden items 'greyed out' in the SceneHierarchy editor.
• to optionally have the display status of entities to affect the render output (possibly only in preview renders, or both preview and batch renders)

[johnhaddon]

We need to discuss how we would like this to work with multiple viewers. Should all viewers share the same set of hidden objects? Or should they potentially have their own set?

Potentially I think we could address differing show/hide settings per viewer by storing the object filter in the context, and then have a context linking ui for sharing/unlinking contexts in different editors.