It's a bit confusing that the created date has to be set when creating a
new note, and the path had to be constructured properly, without which
the note shouldn't be saved. Ideally, an object shouldn't be able exist
in such an inconsistent state.
Also sort the notes based on when last modified. Now that we're moving
towards being a more note-taking app, it makes sense that a recently
modified note goes on top.
Of course, the sorting should be controllable by the user, but the
default probably should be by when modified.
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.
