This add-on adds a basic UI for managing the new VR features to the sidebar, in
a new "VR" tab.
Features:
* Buttons to control general VR session settings (e.g. toggle session,
positional tracking)
* Options for viewport clipping and overlay toggles
* Landmarks - VR base pose management:
** Each landmark can either follow the scene camera or use a custom camera.
** Differentiates between active and selected landmarks. That way landmark
settings can be changed without changing the current reference pose.
** Stored per scene
* Virtual camera gizmo (non-interactive) to show the current VR viewer position
and rotation in regular 3D Views
* Per 3D View feedback options:
** Virtual camera gizmo
** VR mirror (3D View follows VR view position and rotation)
Differential Revision: https://developer.blender.org/D7100
