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
use engravePreview() in snippet/menu.py

Fixes #269.
Merge pull request #270 from dliessi/issue269

use engravePreview() in snippet/menu.py
Merge pull request #267 from dliessi/mac-icon

Mac icon
handle window icon drag on mac os x

please text, I used this information:
http://doc.qt.digia.com/qq/qq18-macfeatures.html#newevents
upd chlog