display path to lilypond command in "engrave custom" dialog

As a LilyPond developer, i have multiple (>20) lilypond builds
in separate directories, with different patches applied etc.
Lilypond version number is not enough to tell them apart in the
"engrave custom" dialog, so i made the actual paths visible
there.  A more elegant solution would be to give builds names,
but i don't think it's worth the trouble.
Merge pull request #168 from janek-warchol/display-path

display path to lilypond command in "engrave custom" dialog