fix normal of surfaces delimited by polylines

See merge request !374

plugin to delete or reverse invisible elements (which can both already be done in the GUI, but not in scripts)

Plugin new view element data

See merge request !373

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