fix import of VRML polys with numvert > 4

The reorganisation of the option panel and the new mesh I/O layer are done.

Mesh I/Os currently available:

- Gmsh MSH format: read and write
- Gmsh POS format for mesh statistics: read and write
- INRIA MESH format: read and write
- STL: read (ascii and binary) and write
- VRML: read (partial) and write
- UNV: write

Everything is significantly faster than before (e.g. readMSH() can be more
than twice as fast when there are lots of physicals)

The rewrite of the geometry and mesh drawing code is complete.

There is still room for optimizations, but mostly at the level of the
algorithms (edge/face generation, striping, etc.)---changes that will
be easy do do thanks to the new much cleaner interfaces.

From preliminary tests (even without new algorithms) the code seems to
be faster (a LOT faster for volume meshes) and to use quite a bit less
memory.

I'm tagging the trunk as "gmsh_2_0_alpha_1".