gnu: Consolidate libjpeg, libpng, and libtiff into one module.

* gnu/packages/image.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add image.scm and remove libjpeg.scm,
  libpng.scm, and libtiff.scm.
* gnu/packages/libjpeg.scm, gnu/packages/libpng.scm, gnu/packages/libtiff.scm:
  Delete files.
* gnu/packages/emacs.scm gnu/packages/games.scm, gnu/packages/gd.scm,
  gnu/packages/geeqie.scm, gnu/packages/ghostscript.scm,
  gnu/packages/gimp.scm, gnu/packages/gnome.scm, gnu/packages/gnunet.scm,
  gnu/packages/gnustep.scm, gnu/packages/gnuzilla.scm,
  gnu/packages/graphviz.scm, gnu/packages/gtk.scm,
  gnu/packages/imagemagick.scm, gnu/packages/inkscape.scm,
  gnu/packages/netpbm.scm, gnu/packages/pdf.scm, gnu/packages/plotutils.scm,
  gnu/packages/qemu.scm, gnu/packages/qt.scm, gnu/packages/scheme.scm,
  gnu/packages/sdl.scm, gnu/packages/slim.scm, gnu/packages/tcl.scm,
  gnu/packages/texlive.scm, gnu/packages/video.scm, gnu/packages/xiph.scm,
  gnu/packages/xorg.scm: Use (gnu packages image) module.