More fixes to the Debian packaging.

As suggested, I removed pdsend, pdreceive and cyclist, and moved the Gem include files into /usr/include/pd-l2ork. The Gem.pc file is edited to reflect this and renamed to pd-l2ork-Gem.pc to avoid file conflicts with pd-extended. Also, I added a preinst/postrm rule to take care of /usr/share/icons/hicolor/128x128/mimetypes/text-x-puredata.png which is also in the pd-extended package. Both pd-extended and pd-l2ork install cleanly on the same system now.