Proposal: Add-on packs

[harry]

I was going to propose adding WxHaskell and/or Gtk2Hs to the platform, but
bloating it with significant dependencies that are only needed for desktop
applications might not be the best approach.

Instead, perhaps OpenGL and GLUT could be moved to a separate add-on pack,
along with WxHaskell and/or Gtk2Hs. This would enable a slimmed-down core
platform for general use, and graphical developers could install the add-on
pack with a full set of GUI packages.

A similar approach is take by .NET and Java, which can be downloaded in
different profiles, depending on the intended usage.