missing line_width/point_size calls

- plugin callbacks now draw their stuff until run/cancel is executed (so we
  manipulate the scene with the additional data displayed)
- made plugin code much shorter
- simpler defaults for StreamLines, CutGrid and Evaluate

This patch fixes an old design flaw in the post-processing module,
namely that we used a list of Post_View objects instead of a list of
pointers to Post_View objects in CTX.post.list.

This had many annoying consequences, in particular the fact that we
needed to be extra careful every time the list was reallocated (as
pointers to the list elements would become invalid).

I tried very hard to change the code everywhere it should be changed,
but I might have missed something. Please let me know if you see
anything suspicious (like a crash when you duplicate/remove/combine/...
post-processing views).

New (very simple, but very nice IMHO) callback mechanism in plugins: a
plugin can now set the min/max/step in input fields, and draw
arbitrary stuff when the values are modified.

This makes the plugin interface MUCH nicer: the plane in CutPlane is
defined interactively (a la "Clipping planes"), the sphere in
CutSphere is visible, the min/max values in CutMap reflect the min/max
in the view, etc.

secured most of the list_pointer accesses

Added automatic documentation generation for plugins

Update copyright notice

Say if the plugin is executed in-place or if it creates new views

The new levelset stuff is too powerful ;-)

- don't cut the timestep in the levelset view if the timestep is
  used to get the field value. I think this makes it less confusing
  (and matches the behaviour of JF's old 'iField' option).

- rename the 'TimeStep' option to 'dTimeStep', also to avoid
  confusing people about which view the time step refers to.

Big Levelset rewrite (+ cleanup of all other plugins).

Not finished yet, but I need to commit this now so that I can use it
with the boss tomorrow at work.

This version should compile and have all the old features working,
plus some extras (levelsets of scalar/vector/tensor 1D, 2D and
3D simplectic views).

JF: 'Plugin(XX).Save' is now deprecated. Use 'Save View[XX] "filename"'
instead.

Reworked the help strings for all the plugins. Easier to understand,
now, Sean? :-)

Fix whitespace in copyright headers

Indented all source files using utils/gmshindent

Updated copyright

better handling of opened files (msh+pos)
min/max computation for msh pos