crop 'out/bin/lilypond' from path displayed in "engrave custom"

this is a follow-up to 59f54df181d3d24be79387eaedec3c89faaa4e31.
Since out/bin/lilypond is in 99.9% cases redundant, lets not
allow it to clutterr the view.
Merge pull request #172 from janek-warchol/display-path

crop 'out/bin/lilypond' from path displayed in "engrave custom"