upgraded all plugins to work with the new post-pro.

This is done "cheaply" for now, by only using old-style list-based datasets.
Taking advantage of the abstraction (so that we can use the plugins on all
datasets) will require more work. The hooks for transparent data translation
are in place, but the actual translation has not been implemented yet.

- 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

copyright update

Update copyright notice

Big Levelset rewrite (+ cleanup of all other plugins).

Not finished yet, but I need to commit this now so that I can use it
with the boss tomorrow at work.

This version should compile and have all the old features working,
plus some extras (levelsets of scalar/vector/tensor 1D, 2D and
3D simplectic views).

JF: 'Plugin(XX).Save' is now deprecated. Use 'Save View[XX] "filename"'
instead.

Fix whitespace in copyright headers

Updated copyright