Small changes inspired by Wednesday's discussions:

- Mac only: Gmsh now publishes its "file types"... That means that
  geo/msh/pos files now have nice icons, and that you can double-click
  on a geo/msh/pos file in the Finder to open it :-)

- removed support for fltk 1.0 (wasn't compiling anymore anyway)

- doubled the timout in GmshServer (2->4s): this seems to help on
  Windows (thanks to jkools@veeco.com)

- added warnings in the docs about the Attractor feature

- increased WB by 1 pixel in the GUI

- replaced more Msg(FATAL) with Msg(ERROR) where it makes sense