move yy globals into Gmsh.l

remove obsolete BGM structure

removing dead code

better export mesh as post-pro view

remesh in script

polish

new "Background Mesh" command

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

- better fallback when outside the octree
- disable bgm when optimizing in netgen

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

make the bgmesh work with quads

shorter view title

set bg gradient by default

- complete file type list
- call Fl_File_Icon::load_system_icons() in the GUI ctor so we have all
  the nice icons and the image preview in the file browser

show stat pane that is appropriate for the current context (instead
of always the mesh stat)

more secure usage of expr

- 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

y off by one

better averaging

typo

make depend

new plugin ExtractElements to extract elements whose values are comprised
in some interval

many fixes in ReadImg (non-square images now work; no default resize; no
default scale change, etc.)

- added little routine to save a post-pro view as a mesh file
- moved all post-pro I/O routines in a separate file

reorder 2D algo menu to match #define numbers (dave)

don't draw mesh nodes by default

make sure entities appear only ONCE in the selection list.

(In order to do this without borking "undo", we have to use
linear searches. So this is slow but it seems to be acceptable
with 5000 entities. If it's really too slow, I will change it
to elimitate duplicates at the end of the selection process.)