Add a camera-tracking icon.

The icon has 3 modes:
1. Enabled, for untracked objects with known positions.
2. Disabled but with the same icon as enabled, for objects being tracked.
3. Disabled and crossed out, for objects without positions.

Note that the selection graphic is able to show un-trackable
"scratch" positions, such as access lines, but the camera icon
will appear disabled for these kinds of objects.