Add orthographic and axis lock features to the 3D view

An orthographic projection of a 3D scene with well-defined axes can sometimes usefully complement our current user-controlled perspective view. For example, this would enable replicating the perspective/front/left/top views of CAD software:

This would be extending the 3D view with options to:

• set the projection to ortho
• partially restrict the view-eye control, e.g. locking it's orientation (the user would retain the ability to pan and zoom)
• fully restrict the view-eye, for a total control of the display

Note that a 3D view with ortho + locked axes is technically similar to our 2D viewer, so extending it instead might enable similar end results.

Potentially relates to:

• #683
• #1510
• #349