unify frescobaldi and frescobaldi-devel in a single Portfile

When Frescobaldi 2.0.11 will be released, the app variant will be
available also in frescobaldi Portfile.
Make frescobaldi-devel a subport to avoid duplicated code.
frescobaldi: use github portgroup
move documentation files to a suitable location. fixes #5
use only xinstall to copy files and create directories in destroot phase

also simplify some details in application bundle creation and make
better use of variables
frescobaldi-devel: upgrade to 20131008