Add option to invert Z axis for orbit-based view controllers

[gerkey]

It's common in certain domains (e.g., aerospace) to construct models and environments with a Z-down coordinate system (it's still right-handed). When visualizing data from such environments, rviz is difficult to use because its camera controls are designed to prefer the ROS-standard Z-up convention. In particular, when working with a Z-down robot model, it's impossible to get an "over-the-shoulder" view of the robot.

This PR adds an orbit view controller option to invert the Z axis to make it easier to work with Z-down environments. To minimize confusion, the option is hidden by the non-orbit based view controllers that don't support it.