licenses.nix: add GNU Affero General Public License version 3

The application "Slic3r" uses this license.
WIP: Add perl modules needed to build "slic3r" (TODO: clean up)

TODO: clean up Wx expression and remove XMLSAXExpatXS which is
incomplete (it wants to write to the nix store to "plug in" itself in
the main SAX parser module) and *optional* for slic3r.
--

Slic3r is a tool to convert a digital 3D model into printing
instructions for your 3D printer. I'll add it in the next commit.

These perl modules are added:
  AlienWxWidgets
  BoostGeometryUtils
  constant-defer
  ExtUtilsCppGuess
  ExtUtilsTypemapsDefault
  ExtUtilsXSpp
  GrowlGNTP
  MathLibm
  MathClipper
  MathConvexHullMonotoneChain
  MathGeometryVoronoi
  MathPlanePath
  ModuleBuildWithXSpp
  NetDBus
  Spiffy
  TestBase
  Wx

A couple of old modules were no longer available on CPAN so I bumped
them:
  DataUUID
  ExtUtilsMakeMaker
  ExtUtilsManifest
  version
  VersionRequirements

XMLTwig fails in the test phase (it is missing a dependencies), so the
test is skipped. How did it pass the test before?
WIP: slic3r: new package

FIXME: slic3r has problems with perl 5.16 due to a locale issue (comma vs
period in floating point numbers).

Slic3r is a G-code generator for 3D printers.