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