graphics/unpaper: Update for 7.0.0
Build system has moved to meson, so rewrite the SlackBuild based on the standard meson template.
The manpage now requires "sphinx" to be built, which is a large python package with many dependencies. Patch out the building of same by default, and add the optional dep of Sphinx and the option SPHINX=true for those that want it.
Update the README file to reflect this build option/optional dep, and re-paragraph it to make it more readable.