Merge branch 'master' of http://gitlab.onelab.info/gmsh/gmsh

what to do in such cases; similar to ThruSections)

- made more OCC_Internals methods private

simpler synchronization logic with GModel: we now keep track of all internal entities that have been unbound
and we always remove them from GModel

We already did the same thing if a new entity was later re-bound to the tag; but we failed to do it if the *same* entity is rebound.
Since we don't guarantee that the subshapes will stay the same, we need to regenerate the GEntity.

We only delete directly in GModel if nothing was changed in the internal CAD models