feat: embed license in executable and add menu item to show it

song.go

@@ -1,6 +1,7 @@
 package sointu
 
 import (
+	_ "embed"
 	"errors"
 )
 
@@ -92,6 +93,9 @@ type (
 	}
 )
 
+//go:embed LICENSE
+var License string
+
 func (s *Score) SongPos(songRow int) SongPos {
 	if s.RowsPerPattern == 0 {
 		return SongPos{OrderRow: 0, PatternRow: 0}