Use libjpeg-turbo as the system-wide default.

ToDo: open an issue for discussion (testing, easy to revert).
Major Xfce refactoring, also adding versions 4.10.*

Please, comment on #296.
xfce: remove xfceutils from release.nix

It's no longer in Xfce.
fontconfig: propagation no longer needed
freetype+fontconfig: add infinality and enable it

Also add fontconfig checks
, change license to "unfree" if using patented code.

Inspired by Corey O'Connor in #89
https://github.com/coreyoconnor/nixpkgs/commit/27fec0aa26d44716134a86efaf48209be7c46138
cairo: bugfix update
gtk2: minor bugfix update
openjpeg: major update
fix the problem of missing expat in a few packages
Revert "openjpeg: major update": not good yet (mupdf)

This reverts commit b2a07ce1894ac8180cfe214ff65ac5ead328c065.
poppler: update; but openjpeg still not detected
openjpeg: use dependencies
libusb1: use pkgconfig
libmtp: update, minor fixes
add poppler_0_20: some pkgs don't build with newer
inkscape: bugfix update
poppler: again downgrade the old version

poppler_0_18 can be removed per-package when they're upgraded
calibre: make build by minor upgrade + new dependencies

0.9.* won't build: https://bugs.launchpad.net/calibre/+bug/1094719
qt47: remove, no longer used
qt48: minor update