vol*: linked orthoviews

[alisterburt]

This is a high value feature for volume visualisation identified in #1086

example:

Linked orthoviews like this enable

• quick navigation to features of interest identified in slices
• precisely defined intersections between mouse and 3D data (like #1092)

Compared to #1092 I think linked orthoviews are more common but users build less intuition for the 3D nature of the data. It's hard to get a feel for where you are in the 3D space.

These are high-value because they are more common and 3D label painting is definitely easier to implement in this type of viewer.