Multiple level crossing sounds

A new optional field called ORTSSoundFileName is made available within the LevelCr blocks of the .w file and a new optional field called ESD_ORTSSoundFileName is made available within the .sd file of the related shape. When such field is present, the related level crossing uses the referenced sound file instead of the standard crossings.sms file.

Discussion: http://www.elvastower.com/forums/index.php?/topic/31210-multiple-crossingsms-files-within-a-route/
Roadmap: https://trello.com/c/lBp2Qqif/346-multiple-level-crossing-sounds