* Enable custom poses for landmarks (so they don't require adding a new
  camera).
* New landmark operators, available in Sidebar menu:
** "Add VR Landmark from Selected Camera"
** "Update Custom Landmark" (updates landmark to match current VR viewer
   pose)
** "Cursor to VR Landmark"
** "Active Camera to Landmark"
** "New Camera from Landmark"
* "Show Landmarks" option, adding gizmos as landmark indicators to 3D
  Views.

This should make the landmarks more practical.

Patch by Sebastian Koenig, with some smaller edits.
Followup commits will do further edits.

Part of T71347.