Read-only sheet + typewriter updates

The sheet supports moving of edit focus to any existing trigger via keyboard arrows and (shift+)tab. The view is read-only, with no support for data updates other than the initial loading of a module. There is no zooming support yet.

The model-related typewriter functionality has been moved from view objects to corresponding model objects. Note and octave names are retrieved from a notation specification in the Share object. The buttons update faster now, although they are still slow with many active notes. There is no keyboard support yet, but it should be straightforward to implement with the new model objects.