better fix: *only* start from scratch if we actually deleted something

make sure to rest the model before doing a add_in_file in the old geo code
(since the parsed command could delete an entity!)

let's add elements after existing ones in regions (instead of copying the
whole vector, overwriting preexisting data); this way we can easily merge
multiple meshes (with duplicate nodes)

enable multiple de-selections at once using the lasso

small simplification + color bug fix

- added 'Delete' button in visibility browser so that we can easily remove
  physicals and partitions from a model

- slightly changed the behaviour of "Save all (discard physicals)" in the msh
  format: now we really discard the physicals, and set the physical-region
  tag to 0 in the msh file. This is the most natural thing to do: this
  way when on reads a msh file, we get the correct information back.

new option to reverse all the normals (for display). Man, the new code rocks :-)

set light on for mesh by default

testing new default color scheme

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".