Feature request: turntable interactor style
In order to facilitate a more orderly/controlled style of viewing 3D data in many cases, a Turntable interactor style is important. Turntable viewport navigation is very similar to trackball navigation, but different in that it keeps the camera roll constant, i.e. the Y-axis is always up.
For alternative description(s), see: Blender stack-exchange: what is the difference between the trackball and turntable style view modes?