Add a straight arpeggio arrow

There have been several requests for an arrow working like the arpeggio but with a straight line.
See : http://musescore.org/en/node/1291 and the attached PDF http://musescore.org/sites/musescore.org/files/arrow.pdf
See also (in french): http://musescore.org/fr/node/12140#comment-65517 with several examples.