Move notebook out of ENNote.
The notebook is really a meta property of the note content, which is what ENNote represents.
This now makes the object system coherent: the ENNote keeps the content of the note bundled
together. The ENNoteRef is an opaque pointer to an actual service note, wherever it lives.
ENNotebook is a reference to an actual service notebook. It thus makes sense that when
creating a new note, you would also hand in an ENNotebook to say where it should live,
and when you go download a note, you only need a note ref. When you search, you get back
the note ref with the notebook as metadata along with title and dates.