Mac OS X: automatically recognize also MacPorts' LilyPond installation
make the engrave command more logical and usable

itself now determines the args to use
in snippet menu call the engraver with the document

otherwise it could use the "sticky" document if that was set
in musicxml import call the engraver correctly
Merge pull request #271 from dliessi/recognize-MacPorts-LilyPond

Mac OS X: automatically recognize also MacPorts' LilyPond installation