Fix post 23402: note dragging in TAB

In TAB's, when notes are dragged, for instance, above an already occupied string, other notes were not re-fretted.

See http://musescore.org/en/node/23402 for an example.
Merge pull request #530 from mgavioli/Fix_post_23402_TAB_note_drag

Fix post 23402: note dragging in TAB