#55 export audio, splitting at significant moments

[Chris Cannam]

A useful option in the export audio dialog would be to export to multiple files, splitting at those moments where an item appears in the currently selected layer. Useful for beat slicing.

Also for beat slicing: a beat detector plugin that can optionally return instants at the starts of the percussive sounds, rather than at the effective onset (i.e. an instant a frame or two sooner, to make it more likely that there will actually be silence at the returned instant).