more robust "Delete Empty Views"

my last commit also fixed several problems in For/EndFor loops:

- the body of the loop was executed at least once, even if the test was
  always false (e.g. For (1:-1:1) )
- the body could be executed one extra time when using optional
  step values
- the for loops didn't work with negative step values

- new parser function GetValue("text", default_value) to query a value
  interactively in a script

- added a nice example using this (to compute slices automatically):
  demos/multislice.script

- new "Delete Empty Views" command
- new View.MinX, View.MinY, View.MinZ, View.MaxX, View.MaxY, View.MaxZ options
- added example on how to do simple volume visualization

better help messages

fix widget position

add color options for elements in views (requested by Philippe a long
time ago)

power law for the alpha channel

add matlab colormaps: hsv, bone, spring, summer, autumn, winter, cool, copper

missing viewchanged=1

typo

add all remaining colormap parameters as options

make the middle button zoom more progressive (+ add acceleration factor
in the GUI)

- new colormaps (jet, hot and pink from Matlab) + generalized
  handling of grayscale colormap

- new View.Colormap option (this enables Gmsh to remember which
  colormap to use or which colormap the current modified colormap
  comes from) + better View.AlphaChannel treatment

- fix saving of color options when color_scheme is != 0

added File->Rename menu

We cannot use _quad as a class name: gcc 2.95 uses it internally

fixed "gcc -pedantic" warnings

Patch from Andreas Jochens so that 'gmsh' can be compiled on amd64
using gcc-4.0.

forgot to add a copy of the simpl pointer in the global simplex tree

Forgot to update the visibility code to work with the new SimplexBase
stuff. Should work again now.

remove extra vol defs

remove extra volume def

add cube example to the demos

add sphere to the demos

fix orientations for Netgen

double precision of middle cross drawing

nicer stippling patterns

Force display of entities during their creation (surfaces during surface creation,
lines during line creation, etc.). This has been requested several times by
various users, so it's probably a good idea. (It can indeed be a bit confusing
when one creates a new surface and nothing changes on the screen after pressiong
'e'.)

update to latest gl2ps