Added plugin for visualizing discretization error (co-written by Thomas Bolemann from the IAG at Stuttgart university and I).

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is then available to the plugin) -- for Koen

(useful for simulating synthetic foam-like materials)

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- work on plugin user-friendliness (short help & co)

in a force field (should be generalized e.g. for Lorentz force,
but that a TODO ;-)

new Tetrahedralize plugin

- moved GUI to FlGui + added experimental QGui to test Qt

- fixed most MSVC and gcc compiler warnings

- converted some remaining tabs to whitespace (that's why the patch is
  so large; please try to remember to indent with 2 spaces and not
  tabs when you commit stuff to CVS)

- merged modified patch from Konstantinos Poulios to allow saving
  physical/partition as element tags in mesh formats that don't
  support them natively