New options to enable/desable all default post-processing/solver plugins

proof of concept for unproject (e.g. to add points with the mouse)

fix gcc warning + add "cancel" button to make the dialog look like
the other ones

Say we didn't find any solver plugins if GMSHPLUGINSHOME is not defined.

This is not perfect, but it allows us to *not* display the annoying
popups for "regular" users, while still registering JF's solver plugin...

allow x,y,z in expressions

new solver options: SocketCommand and NameCommand (patch by
<C.Stott@surrey.ac.uk>)

small fixes for last commit:

- only delete view.adaptive if we really delete the view (we might want to
  use the adaptive data in a duplicate view)

- add missing opt_view_global_zoom in update_view()

- don't even try to open StructuralSolver's data files if GMSHPLUGINSHOME
  is not defined

New high order interpolation.

JFR

draw the labels a little bit closer to the entities

subclass the original file chooser and move it into the GUI class, so
that we can set its position (the FLTK folks don't want to patch the
standard file chooser, so this is the only solution...)

fix problem with new dynamic menus and plugin dialogs (unfortunately, this
re-adds the (small) memory leak in the GUI)

- the main menu is now scrollable when there are more than 25
buttons and it can handle an arbitrary number of buttons (this removes
the "100 views max in the GUI" limitation)

- the view number is now also displayed in the menu (to make it easier
to find the view when one has many many views)

- added "Remove empty views" menu item

add missing #ifdef HAVE_FLTK

make code compile on Mac again

re-add return statements in functions returning non-void. JF: my compiler
wants this (and he's right)

make depend

- fixed a couple of gcc warnings + added missing o->end() in
  jf's StructuralSolver plugin

- fixed the view option code: FLTK > 1.1.5rc1 breaks our use of
  widget->changed()...

- added background mesh support in the aniso algorithm

little hack to visualize overlapping multi-patch spheres with constant
offsets (use timeStep<0...)

make depend

- Triangle now supports our background meshes

- fixed memory leak in Optimize_Netgen (forgot to free the volume
  vertices) + added missing 2nd order reset

- fixed a couple of typos in the docs

set a parameter for the max number of elements per bucket
(default=100, which seems to be a good memory vs. speed trade-off for
meshes up to 1/2 million elements)

- allow more than 2 elements per bucket for ST!
- add missing tensor simplex case

- fixed seg fault in Plugin(Triangulate)
- added BBox computation in plugins when the coordinates are changed

mention explicitly if a plugin only works for simplices

Generalized Plugin(Triangulate) to handle vector and tensor views

- mesh.algo -> algo2d and algo3d
- replaced most of the remaining radio buttons with Fl_Choices (requires
  less screen real estate)

reset the input lists (do not delete them)

forgot to destroy the evaluators

nbU -> nPointsU

replace CutCircle/CutCurve with CutParametric

- fixed out of bound access in OctreePost for multi-step views

- added dView option to StreamLines plugin to interpolate an
  arbitrary scalar view on the stream lines

- set VectorType to Displacement in Plugin(StreamLines)
- shout when a plugin option/name does not exist