Various Makefile improvements, icon license clarifications and new svg icon

I added the SVG version of Isodumper's icon plus instructions in the Makefile and spec file to get it installed. The license and credits of the icon is now specified in the README.md, and a copy of the LGPLv2.1+ has been included (the COPYING has been moved to COPYING.GPL for clarity).

The clean section of the Makefile has been updated, and the tar section now builds a .tar.xz tarball.