add patch version in API version: it's not strictly necessary (we don't break the API between patch releases), but it makes things simpler.

rationale: with non-breaking API changes (e.g. adding a new function) one had to use gmsh/option/getNumber("General.Version") to check the Gmsh version; using the same numbering for the API and for Gmsh means one can simply examine the API version.

Fixes #997