Specify live camera input in image menu rather than per scene

Rather than having the line `"liveCamera": false` (or true) at the top of the scene, have it be a field in each image menu.

It is possible to have any number of image menus in a scene, and it's possible some will allow live camera while some will not.

Start by locating the scene representation and the image-menu handling for the `liveCamera` field. Trace how the scene-level setting currently controls camera access, then determine how each image menu can carry its own setting. Done means different image menus in one scene can independently allow or disallow live camera input.

Written by the indexing model from the issue text.