Various bug fixes for Verovio and adding d3.js for current measure highlighting

The fixes include:
- changing the file extension from .xml to .mei for getting a string (and not an XMLDocument)
- updating Verovio to the latest version which implements the getPageWithElement method
- adding d3.js for highlighting the measures
- modification of FreiDi_audio.js in order to return the measureID
- adding a version of the MEI files without the facsimile subtree (unused for rendering with Verovio)