fix #248
some versions of Frescobaldi's icon for Mac OS X

frescobaldi-shadow.svg: most Mac hi-res icons have a shadow
frescobaldi-bigF.svg, -thickF.svg: "F" more visible in low-res versions
Mac .icns icons file

1. export frescobaldi-shadow.svg to PNG at sizes 512, 256, 128
2. export frescobaldi-bigF.svg to PNG at size 32
3. export frescobaldi-thickF.svg to PNG at size 16
4. create frescobaldi.icns from the PNGs with Apple Icon Composer
be sure include mac icons in source dist
Mac OS X: automatically recognize also MacPorts' LilyPond installation
globbing not needed when constructing a search path
oops