wmcdplay: Install art script.

wmcdplay: Install art script.

Changes include:
- The artwork files XPM/*.art are now created during build time and
  therefore have been removed from the repository.
- The script which creates them is now installed alongside the art files in
  case users would like to design their own artwork.
- The "makeall" script, whose function has been replaced by the Makefile,
  has been removed.
- The default artwork can now be chosen at compile time by the --with-art
  option to ./configure.
- Changes have been documented in ARTWORK.

Inspired in part by the Debian patch [1].

[1] http://sources.debian.net/src/wmcdplay/1.0beta1-13/debian/patches/install_art_scripts.patch/