put origin world coordinate at center viewport + make sure model does not move when changing rotation center

The first modification (concerning origin world coordinate) affects models whose pseudo center of mass is not at (0, 0, 0). Sadly, for those model, opt files that change the translation will not produce the same result than before these commits. Those opt files have to be updated...

To compare modification origin world coordinate:

1. Open model whose bounding box is far from the origin (or open squareFar.geo)
2. Zoom at an arbitrary point
3. Open manipulator window and manually dezoom

To compare modification rotation center:

1. Open model
2. Rotate model
3. Open Options window > General > General
4. Play with "Rotate around pseudo center of mass" and "Rotation center"