Add a module for MiniDLNA

Now my NixOS machine can serve videos to my Bluray player!
add xbmc window manager
modules/config/fonts.nix: export GDFONTPATH in /etc/profile

Set the environment variable GDFONTPATH to the list of directories where fonts
are expected to be installed. This is search path is used by same GD-based
applications, such as Gnuplot.

Alternatively, we could rely on fontconfig to achieve the same thing, i.e. by
running

  for n in $(fc-list | sed -r -e 's|^([^:]+):.*$|\1|'); do echo $(dirname "$n"); done | sort | uniq

to get that list at run-time, but the static approach feels more deterministic.
modules/config/fonts.nix: revert "export GDFONTPATH in /etc/profile"

This reverts commit 2802538dc2e2a8923e3d3cc3e144abc4e1561760. The idea
was good, but unfortunately $GDFONTPATH has a different format, i.e.
programs that use this variable don't descend into the directories
listed here like fontconfig does, so we cannot use the same values for
both settings.
Merge pull request #90 from iElectric/xbmc

Add xbmc window manager