More "airplane" work...

- rewrote the way we handle lights: glEnable(GL_LIGHTING) is now used
  at the lowest level, where it belongs. This fixes many lighting
  glitches, and should make the whole stuff much more predictable;

- simplified the display modes for geometry and mesh: the choice
  between wireframe/solid is now independent of the choice of
  lighting/no lighting (and there is a new "Enable lighting" option
  for the geometry, too--even if it does only affect things like
  normals/tangents at the moment);

- brand new code to draw really nice (shaded) 3D arrows + 3 options
  that fully parameterize them (they can degenerate into pyramids,
  cones, w/ or w/o stems, etc.);

- new options so that we can also use the new arrows outside the
  post-processing module (e.g. for tangents/normals).

- "alt+d" now simply switches between solid and wireframe mode;

- new shortcut "alt+w" to switch the lighting mode for all the modules
  (geo/mesh + all post views) at once.

bring option docos up-to-date

Add DisplacementFactor option + rename ArrowScale->ArrowSize and
ArrowType->VectorType

Small corrections

Corrections apres relecture / part 1

More work on chapters 1 and 2. Should be mostly OK now.

Major rewrite of the Overview section + web page

small updates

Added "gmsh -doc" to automatically generate the texinfo documentaion
for all the options.