debian/copyright: correct cc0 links

Fix incorrect link to CC0 deed and legal code.
debian: split out libmypaint (UNREL), rules tweaks

debian/control: update homepage link

debian/control: provide Vcs-* info

debian/control: split out libmypaint, libmypaint-dev, libmypaint-brushes
                - these may not be the final package names

debian/rules: use dh_install, and use it to check for overlooked files