Add support for axis-aligned (front, side) "flat" display-only images

[adamkewley]

Estimate: 2-6d (adding texture support is easy, most of the work is around adding relevant dialog boxes, buttons, etc.)

For model building.

Let the user add an image--e.g. an arm anatomy diagram--along an axis of the scene /w adjustable opacity, so that the user can edit the OpenSim model with that as a ("tracing") background. This is really handy for sculpting, where people usually work this way in other software (e.g. Blender).