* Refactor Layer classes so as no longer to store a single View pointer; instead they need to be able to draw themselves on any View on demand. Layers with caches (e.g. spectrogram) will need to be further refactored so as to maintain a per-View cache * Begin refactoring MainWindow by pulling out the document stuff (set of layers, models etc) into a Document class. Not yet in use. This revision is fairly unstable.