added interactive callbacks in CutParametric and Annotate

Generalized the Levelset routines so that we can compute isovolumes with
all levelset-based plugins. This allows to extract either side of a
levelset (i.e., a half space with CutPlane, the interior/exterior of
a sphere with CutSphere, or the isovolume inside an isosurface with
CutMap).

copyright update

removed unused extern

- cleaned up + generalized font selection
- generalized Plugin(Annotate) to allow font style selection for
  each text fragment
- fixed bug in per-view color selection

- new alternative Draw_String function taking a "style" argument (used
  only for font size at the moment)
- new Plugin(Annotate) to add simple text strings to a view