Finally we can stop using the weird convention in 3D views (where
X grows to the east, Z grows to the south, Y goes up) that caused
endless trouble having to keep in mind that the coordinates in 3D
views are rotated compared to the map coordinates. I am not sure
anymore why I chose to use this convention, but most likely because
Qt's plane mesh is oriented this way (and also various other
3D software has scene with Y going up).

Anyway, the world coordinates are not flipped anymore, so the axes
are consistent with the map coordinates 🎉 - i.e. X grows to
the east, Y grows to the north, Z goes up (well, at least that's
the case for many commonly used CRS). The 3D world coordinates
are now only translated by the scene's origin point compared to
map coordinates.