frescobaldi: don't add Python version suffix to binary name
frescobaldi: use python.prefix instead of manually writing the path
frescobaldi: link man page from Python.framework tree instead of copying

Fixes #8.
frescobaldi: avoid duplicating directory names
frescobaldi: install icon and desktop files inside ${prefix}/share

Fixes #7.
frescobaldi: remove useless white space
frescobaldi: revision bump
README.md: update upgrade instructions.