Remove the "app default notebook" machinery, which is effectively obviated by SNA tokens.
Move note content to its own class "cluster", ENNoteContent.
This removes the weird constraint that the content must be set at init time of the note and never after.
Note content objects are immutable once created. Currently plaintext and HTML subclasses available.
Advanced users get the base class (pure ENML) variant if they know what they're doing.