* Add command history class, and basic undo/redo menus. No actual commands to undo/redo yet. Selecting the placeholders sometimes seems to cause a crash, so this looks a little uncertain so far. * Add Rename Layer * Remove models from playback when their layers are removed (and ref counts hit zero) * Don't hang around waiting so much when there's work to be done in the audio buffer fill thread * Put more sensible names on layers generated from transforms * Add basic editing to time-value layer like existing editing in time-instants layer, and make both of them snap to the appropriate resolution during drag