merging offscreen rendering from branch

- fix discrete surf (for the 100000th time)
- new Status N command in parser to force mesh status
- create reverse discrete curves

reverting last change: Matthias' fix does not work

trying Matthias Melcher's fix for the delete widget crashes (this fix
does not require to derive our own popup button class)

minimize the number of files that include GmshVersion.h: it's annoying to
recompiletoo many files just because the version string changes

fix popup+make depend

moved most OS-dependent functions in OS.cpp

"pure windows" version (without cygwin) is almost there: the solver
interface now works quite nicely using native Windodiows sockets and
the timers work too. There are still some bugs with binary files and
user-created functions in the parser.

- rename CLIENT_VIEW into CLIENT_MERGE_FILE
- new CLIENT_PARSE_STRING

added support for empty lists (aa[] = {};) and concatenation of lists
(aa[] += {1,2,3};)

newsl and newll to get next available loop numbers

move yy globals into Gmsh.l

remesh in script

new "Background Mesh" command

removed the STEP parser: it's unusable, it makes the parser bigger,
and it introduces conflicts in the syntax.

Rewrote the background mesh code to use the new post-pro octree instead
of the old brick stuff. It's 100x faster ;-)

- if FLTK provides the image libraries, use them (and don't test by ourselves)
- added new (fltk-based) image reader routines for JPEG, PNG and BMP

fixed ugly extrusion syntax (finally!)

extrusions are now specified in the same way as all other transformations
(the old syntax is still available, but is deprecated)

rewrote the smooth normal routines:

- the averaging is now done by clusters, which is much better for
  geometries that have sharp edges  (it's a little slower, but the
  visual results are much better)
- there is now only a single smooth normal container in the mesh
  (instead of one per surface)

- new button under the graphic window to temporarily disable mouse
  selection (speeds-up redrawing of very large models + permits to
  rotate/zoom-in a model in selection mode even when the whole screen
  is full of selectable entities--e.g. a surface mesh)

- new "lasso" selection mode (to select entities using the same kind
  of lasso as the lasso zoom: just Ctrl+click, then drag the mouse in
  selection mode; the shortcuts are the same as for the lasso zoom)

- it is now possible to unselect entities using the middle mouse button
  (only for the creation of physicals at the moment; not sure if it's
  useful in the other cases)

- new button in visibility browser to invert the current selection
  (very useful e.g. when multiple physical entities are associated
  with a given elementary entity, in order to "peel" away the model
  when adding new physicals; cf. philou)

- changed meaning of Escape shortcut (cancel lasso or toggle mouse
  selection) + restore standard fltk Escape handling for all dialog
  windows

- updated copyright string

- new mesh label mode (coordinates); all label types are now also
  available for mesh vertices

- added option in 'Print Option' dialog to disable printing of help
  strings

- added a comment string with the date when creating a new file

- new snapping grid for adding points in the GUI

- new button under the graphic window to temporarily disable mouse
  selection (speeds-up redrawing of very large models + permits to
  rotate/zoom-in a model in selection mode even when the whole screen
  is full of selectable entities--e.g. a surface mesh)

- new "lasso" selection mode (to select entities using the same kind
  of lasso as the lasso zoom: just Ctrl+click, then drag the mouse in
  selection mode; the shortcuts are the same as for the lasso zoom)

- it is now possible to unselect entities using the middle mouse button
  (only for the creation of physicals at the moment; not sure if it's
  useful in the other cases)

- new button in visibility browser to invert the current selection
  (very useful e.g. when multiple physical entities are associated
  with a given elementary entity, in order to "peel" away the model
  when adding new physicals; cf. philou)

- changed meaning of Escape shortcut (cancel lasso or toggle mouse
  selection) + restore standard fltk Escape handling for all dialog
  windows

- updated copyright string

- new mesh label mode (coordinates); all label types are now also
  available for mesh vertices

- added option in 'Print Option' dialog to disable printing of help
  strings

- added a comment string with the date when creating a new file

- new snapping grid for adding points in the GUI

Removed all compile-time depdencies on byte ordering (big- or little-
endianness). Replaced them with run-time conversions where necessary (only
for packing/unpacking of colors into unsigned ints). Performance hit
should be negligeable, but this will allow to compile Universal Binaries
on Mac in a single step (instead of having to compile the PPC/big-endian
and the i386/little-endian separately, which wouls be a royal pain in the
butt).

make depend

trying to fix the transfinite volume bug introduced by the 'left' and
'alternate' transfinite surface changes made a while ago

make "BoundingBox{x,X,y,Y,z,Z}" actually do what the documentation claims
it does

Don't reinitialize the symbol tree in OpenFile.

(We don't reinitialize the option database, so why should we reinitialize
the variables? Using a single global symbol tree we can again define
variables with -string "" on the command line.)

cleanup STL remesher GUI

moving all external code to contrib subdirectory

use it now with -partition N for N partitions
only works with tets and triangles
output should be added (useless for now)

explain #list[]