Now when adding/editing/removing a note, it gets modified from the
directory it was present in. There is no longer a just a plain list of
all notes, but always a tree of notes, which are inside Folders.
Also refactor this a bit so Note contains a NoteData. I'm really not
convinced about today's abstractions, and feel I might have made this
entire thing much much worse.
The loading/saving/deleting of the Note class is now inbuilt. It's not
offloaded to a NoteRepo. We're always going to be storing them on disk,
for now. It's also way easier to manage them this way.
