* Refactor sparse models. Previously the 1D and time-value models duplicated a lot of code; now there is a base class (SparseModel) templated on the stored point type, and the subclasses define point types with the necessary characteristics. * Add NoteModel, a new SparseModel subclass. * Reorganise local feature description display. Instead of asking the layer to draw its own, just query it for a textual description and draw that in Pane. Greatly simplifies this part of the layer code. * Add local feature descriptions to colour 3D plot and waveform layers. * Add pitch in MIDI-pitch-and-cents to spectrogram layer. * Give AudioGenerator its own mutex to shorten lock times in CallbackPlaySource. * Minor adjustments to layers menu &c